refactor: session-based external auth, dynamic sources, drop netease UID

Auth (external API):
- ID: 16-digit random (non-sequential); Secret: SeaReport- + 32 hex
- POST /auth/session: ID+Secret -> Bearer SESSION (24h, single-session,
  old session invalidated on re-issue, disabled client invalidates)
- clientAuth now validates Bearer SESSION via api_sessions JOIN api_clients

Sources (dynamic, no default, open-source friendly):
- sources table + CRUD route (/api/sources, owner; delete guarded by usage)
- users/user_identities.source ENUM -> VARCHAR, seeded netease/skin
- register/admin create/identity bind: validate against enabled sources
- UI: 来源管理 page; source dropdowns loaded dynamically everywhere
  (register, dashboard identity, users admin, bans), labels dynamic

UID removal:
- game_uid/reporter_game_uid no longer required (db default '', validations
  dropped, frontend fields optional)

Docs: EXTERNAL-API.md session flow + new credential format; API.md updated
Verified: 37 checks (syntax, session logic, source CRUD, UID removal, docs)

public/index.html

@@ -76,6 +76,7 @@
   <script src="js/pages/servers.js"></script>
   <script src="js/pages/bans.js"></script>
   <script src="js/pages/logs.js"></script>
+  <script src="js/pages/sources.js"></script>
   <script src="js/pages/features.js"></script>
   <script src="js/pages/feature-detail.js"></script>
   <script src="js/app.js"></script>