Refactor auth initialization to support three config states:
1. DB config (auth_provider_config table) — primary source
2. OIDC_* env vars — fallback when DB config absent
3. Unconfigured — graceful handling when neither source available
Changes:
- auth.ts: Add initAuth() async factory, getAuth() getter, getAuthPromise()
- index.ts: Call initAuth() at startup before serve()
- middleware/auth.ts: Use getAuth() instead of direct auth import
- Add auth.test.ts covering all three config states
Preserves AUTH_DISABLED=true behavior and original hairpin NAT pattern.
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