Fix cross-origin API access, bcrypt login break, and weak secret key

The frontend only worked from localhost:3000 — the browser made absolute
cross-origin calls to localhost:8000, which (a) points at the visitor's own
machine when accessed via the LAN IP or Cloudflare tunnel, and (b) was blocked
by CORS (backend only allowed localhost:3000). Now all API calls are
same-origin and proxied to the backend:

- api.ts: default API base to same-origin ("") instead of localhost:8000
- docker-compose: NEXT_PUBLIC_API_URL="" (client uses relative /api)
- next.config.ts: server-side rewrite uses API_INTERNAL_URL (http://backend:8000
  inside Docker) so :3000 direct access proxies correctly
- main.py: broaden CORS via allow_origin_regex (localhost + private LAN) and an
  optional CORS_ORIGINS env var, as defense-in-depth for direct :8000 access

Login was returning 500: passlib 1.7.4 is incompatible with the bcrypt 5.0.0
actually installed in the image (requirements pin 4.1.2, but the image drifted).
- security.py: call bcrypt directly, truncating to 72 bytes; existing $2b$
  hashes verify unchanged, and it works under both bcrypt 4.x and 5.x

Security: SECRET_KEY was a known placeholder string while the app is exposed on
the LAN and via Cloudflare tunnel — anyone could forge admin JWTs. Regenerated
to a strong random value in backend/.env (gitignored; not committed).

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

backend/app/core/config.py

@@ -13,6 +13,7 @@ class Settings(BaseSettings):
 
     SECRET_KEY: str = "dev-secret-key-change-in-production"
     ALGORITHM: str = "HS256"
+    CORS_ORIGINS: Optional[str] = None  # comma-separated extra allowed origins
     ACCESS_TOKEN_EXPIRE_MINUTES: int = 480
 
     AI_PROVIDER: str = "openai"