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/models/UsageAlertDelivery.ts

@@ -0,0 +1,21 @@
+import mongoose from 'mongoose'
+
+export interface UsageAlertDeliveryDocument extends mongoose.Document {
+  monthKey: string
+  thresholdUsd: number
+  costUsd: number
+  recipient: string
+  sentAt: Date
+}
+
+const usageAlertDeliverySchema = new mongoose.Schema<UsageAlertDeliveryDocument>({
+  monthKey: { type: String, required: true, unique: true },
+  thresholdUsd: { type: Number, required: true },
+  costUsd: { type: Number, required: true },
+  recipient: { type: String, required: true },
+  sentAt: { type: Date, default: () => new Date() },
+})
+
+export const UsageAlertDelivery =
+  (mongoose.models.UsageAlertDelivery as mongoose.Model<UsageAlertDeliveryDocument> | undefined) ||
+  mongoose.model<UsageAlertDeliveryDocument>('UsageAlertDelivery', usageAlertDeliverySchema)

server/models/UsageEvent.ts

@@ -0,0 +1,45 @@
+import mongoose from 'mongoose'
+
+const { Schema } = mongoose
+
+export type UsageKind = 'stt' | 'tts' | 'llm'
+export type UsageProvider = 'openai' | 'speaches' | 'piper' | 'cache'
+
+export interface UsageEventDocument extends mongoose.Document {
+  user?: mongoose.Types.ObjectId
+  sessionId?: string
+  kind: UsageKind
+  provider: UsageProvider
+  model: string
+  endpoint: string
+  /** Audio length for STT in seconds */
+  audioSeconds?: number
+  /** Synthesized text length for TTS */
+  characters?: number
+  inputTokens?: number
+  outputTokens?: number
+  /** Estimated cost in USD (0 for self-hosted providers and cache hits) */
+  costUsd: number
+  createdAt: Date
+}
+
+const usageEventSchema = new mongoose.Schema<UsageEventDocument>({
+  user: { type: Schema.Types.ObjectId, ref: 'User', index: true },
+  sessionId: { type: String },
+  kind: { type: String, enum: ['stt', 'tts', 'llm'], required: true },
+  provider: { type: String, enum: ['openai', 'speaches', 'piper', 'cache'], required: true },
+  model: { type: String, required: true },
+  endpoint: { type: String, required: true },
+  audioSeconds: { type: Number },
+  characters: { type: Number },
+  inputTokens: { type: Number },
+  outputTokens: { type: Number },
+  costUsd: { type: Number, required: true, default: 0 },
+  createdAt: { type: Date, default: () => new Date(), index: true },
+})
+
+usageEventSchema.index({ user: 1, createdAt: -1 })
+
+export const UsageEvent =
+  (mongoose.models.UsageEvent as mongoose.Model<UsageEventDocument> | undefined) ||
+  mongoose.model<UsageEventDocument>('UsageEvent', usageEventSchema)