feat(server): per-user AI usage tracking, cost alerting, and endpoint hardening

Usage tracking:
- new UsageEvent collection records every STT/TTS/LLM call per user with
  provider, model, volume (audio seconds, characters, tokens) and an
  estimated USD cost; self-hosted providers (Speaches/Piper) and cache
  hits record at $0
- pricing table for whisper-1, tts-1, gpt-5-nano & co. in server/utils/usage.ts
- weekly KPI mail gains an "AI-Nutzung & Kosten" section: weekly and
  rolling 30-day cost, per-kind breakdown, top 5 users by cost
- quota alert mail when rolling 30-day cost exceeds USAGE_ALERT_USD
  (default $5), at most once per calendar month (UsageAlertDelivery)

Hardening:
- /api/atc/say now requires an authenticated session (middleware
  exemption removed); useFlightLabAudio sends the bearer token
- /api/service/tools/latency requires auth (was a public LLM endpoint)
- per-user rate limits: PTT 20/min, say 60/min, latency 5/min
- cron endpoints (waitlist-drip, weekly-kpi-report) require a shared
  secret via ?secret= or x-cron-secret (CRON_SECRET, falls back to
  KPI_CRON_SECRET); allowed with a warning while unset so existing
  deployments keep working
- PTT records the actual transcribed audio duration for billing accuracy

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/api/atc/ptt.post.ts

@@ -1,6 +1,6 @@
 // server/api/atc/ptt.post.ts
 import { createError, readBody } from "h3";
-import { writeFile, rm } from "node:fs/promises";
+import { writeFile, rm, readFile } from "node:fs/promises";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 import { randomUUID } from "node:crypto";
@@ -9,6 +9,8 @@ import { getOpenAIClient } from "../../utils/openai";
 import { createReadStream } from "node:fs";
 import { TransmissionLog } from "../../models/TransmissionLog";
 import { getUserFromEvent } from "../../utils/auth";
+import { enforceRateLimit, getClientIp } from "../../utils/rateLimit";
+import { recordUsage } from "../../utils/usage";
 
 type AudioFormat = 'wav' | 'mp3' | 'ogg' | 'webm'
 
@@ -66,6 +68,13 @@ function decodeAudioPayload(encoded: string): Buffer {
     return buffer;
 }
 
+function wavDurationSeconds(buffer: Buffer): number | undefined {
+    if (buffer.length < 44 || buffer.toString('ascii', 0, 4) !== 'RIFF') return undefined;
+    const byteRate = buffer.readUInt32LE(28);
+    if (!byteRate) return undefined;
+    return Math.round(((buffer.length - 44) / byteRate) * 100) / 100;
+}
+
 async function convertToWav(inputPath: string, outputPath: string) {
     await sh("ffmpeg", [
         "-y", "-i", inputPath,
@@ -76,7 +85,12 @@ async function convertToWav(inputPath: string, outputPath: string) {
     ]);
 }
 
+const PTT_RATE_LIMIT_PER_MINUTE = 20;
+
 export default defineEventHandler(async (event) => {
+    const user = await getUserFromEvent(event);
+    enforceRateLimit(event, 'atc-ptt', user ? String(user._id) : getClientIp(event), PTT_RATE_LIMIT_PER_MINUTE);
+
     const body = await readBody<PTTRequest>(event);
 
     if (!body.audio || !body.moduleId || !body.lessonId) {
@@ -115,6 +129,34 @@ export default defineEventHandler(async (event) => {
 
         const transcribedText = transcription.text.trim();
 
+        // Audio length for usage accounting. The Whisper input is 16kHz mono WAV
+        // in the normal path; fall back to a byte-rate estimate if header parsing fails.
+        let audioSeconds: number | undefined;
+        try {
+            const wavBuffer = audioFileForWhisper === tmpAudioInput && format === 'wav'
+                ? audioBuffer
+                : await readFile(audioFileForWhisper);
+            audioSeconds = wavDurationSeconds(wavBuffer) ?? Math.round((wavBuffer.length / 32000) * 100) / 100;
+        } catch {
+            audioSeconds = Math.round((audioBuffer.length / 32000) * 100) / 100;
+        }
+
+        // Prefer the explicit top-level sessionId (Python backend session).
+        // Fall back to the legacy context.flags.session_id for older clients.
+        const sessionId = body.sessionId
+            ?? (typeof body.context?.flags?.session_id === 'string' ? body.context.flags.session_id : undefined);
+
+        // Whisper bills the audio even when nothing was recognized.
+        await recordUsage({
+            user: user?._id ? String(user._id) : undefined,
+            sessionId,
+            kind: 'stt',
+            provider: 'openai',
+            model: 'whisper-1',
+            endpoint: '/api/atc/ptt',
+            audioSeconds,
+        });
+
         if (!transcribedText) {
             throw createError({ statusCode: 400, statusMessage: "No speech detected in audio" });
         }
@@ -125,12 +167,6 @@ export default defineEventHandler(async (event) => {
         }
 
         try {
-            const user = await getUserFromEvent(event);
-            // Prefer the explicit top-level sessionId (Python backend session).
-            // Fall back to the legacy context.flags.session_id for older clients.
-            const sessionId = body.sessionId
-                ?? (typeof body.context?.flags?.session_id === 'string' ? body.context.flags.session_id : undefined);
-
             await TransmissionLog.create({
                 user: user?._id,
                 role: "pilot",

server/api/atc/say.post.ts

@@ -7,6 +7,9 @@ import {normalize, TTS_MODEL, normalizeATC} from "../../utils/normalize";
 import { getServerRuntimeConfig } from "../../utils/runtimeConfig";
 import {request} from "node:http";
 import { TransmissionLog } from "../../models/TransmissionLog";
+import { requireUserSession } from "../../utils/auth";
+import { enforceRateLimit } from "../../utils/rateLimit";
+import { recordUsage } from "../../utils/usage";
 
 
 function outDir() {
@@ -157,7 +160,12 @@ async function speachesTTS(
     return Buffer.from(arr);
 }
 
+const SAY_RATE_LIMIT_PER_MINUTE = 60;
+
 export default defineEventHandler(async (event) => {
+    const user = await requireUserSession(event);
+    enforceRateLimit(event, 'atc-say', String(user._id), SAY_RATE_LIMIT_PER_MINUTE);
+
     const runtimeConfig = getServerRuntimeConfig();
     const body = await readBody<{
         text?: string;
@@ -177,8 +185,6 @@ export default defineEventHandler(async (event) => {
         preNormalized?: boolean;
     }>(event);
 
-    // const user = await requireUserSession(event);
-
     const rawSessionId = typeof body?.sessionId === "string"
         ? body.sessionId.trim()
         : "";
@@ -348,9 +354,20 @@ export default defineEventHandler(async (event) => {
             }
         };
 
+        await recordUsage({
+            user: String(user._id),
+            sessionId,
+            kind: 'tts',
+            // Cache hits cost nothing regardless of which provider filled the cache.
+            provider: cacheHit ? 'cache' : ttsProvider,
+            model: modelUsed,
+            endpoint: '/api/atc/say',
+            characters: normalized.length,
+        });
+
         try {
             await TransmissionLog.create({
-                // user: user._id,
+                user: user._id,
                 role: "atc",
                 channel: "say",
                 direction: "outgoing",

server/api/service/tools/latency.get.ts

@@ -2,6 +2,9 @@
 import { createError } from 'h3'
 import { getOpenAIClient } from '../../../utils/openai'
 import { getServerRuntimeConfig } from '../../../utils/runtimeConfig'
+import { requireUserSession } from '../../../utils/auth'
+import { enforceRateLimit } from '../../../utils/rateLimit'
+import { recordUsage } from '../../../utils/usage'
 
 const SYSTEM_PROMPT =
     'Check if the pilot readback contains ALL of: Frankfurt or EDDF, FL320, and 120.8 MHz. ' +
@@ -10,7 +13,10 @@ const SYSTEM_PROMPT =
 const READBACK =
     'Lufthanser four seven eight cleared fra via NORDA1A, climb 5000 feet, expect flight level tree too zero, dep 120 decimal 8, squawk 4213.';
 
-export default defineEventHandler(async () => {
+export default defineEventHandler(async (event) => {
+    const user = await requireUserSession(event)
+    enforceRateLimit(event, 'tools-latency', String(user._id), 5)
+
     const client = getOpenAIClient()
     const { llmModel } = getServerRuntimeConfig()
     const model = llmModel || 'chatgpt-5-nano'
@@ -32,6 +38,16 @@ export default defineEventHandler(async () => {
         const validResult = Number.isInteger(parsed) && parsed >= 0 && parsed <= 2 ? parsed : null
         const latencyMs = Date.now() - started
 
+        await recordUsage({
+            user: String(user._id),
+            kind: 'llm',
+            provider: 'openai',
+            model,
+            endpoint: '/api/service/tools/latency',
+            inputTokens: response.usage?.prompt_tokens,
+            outputTokens: response.usage?.completion_tokens,
+        })
+
         return {
             result: validResult,
             raw,