增加常量文件，修改作者注释

db/20250427232209_create_users_table.down.sql

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+drop sequence public.users_cid_seq;
+
+drop table public.users;

db/20250427232209_create_users_table.up.sql

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+create sequence public.users_cid_seq
+    as integer;
+
+create table public.users
+(
+    cid            serial
+        constraint users_pk
+        default nextval('users_cid_seq')
+        primary key,
+    password       char(60) not null,
+    first_name     varchar(255),
+    last_name      varchar(255),
+    network_rating smallint not null
+);

db/20250515185717_create_config_table.down.sql

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+drop table config;

db/20250515185717_create_config_table.up.sql

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+create table config
+(
+    key   varchar not null,
+    value varchar not null
+);
+
+create unique index config_key_uindex
+    on config (key);