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/security.py

@@ -1,21 +1,34 @@
 from datetime import datetime, timedelta
 from typing import Optional, Any
+import bcrypt
 from jose import JWTError, jwt
-from passlib.context import CryptContext
 from fastapi import HTTPException, status, Depends
 from fastapi.security import OAuth2PasswordBearer
 from app.core.config import settings
 
-pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
 oauth2_scheme = OAuth2PasswordBearer(tokenUrl=f"{settings.API_V1_STR}/auth/login")
 
+# We call bcrypt directly rather than through passlib: passlib 1.7.x is
+# incompatible with bcrypt >= 4.1 (its version shim raises on the modern
+# library). bcrypt only uses the first 72 bytes of a password, so we truncate
+# to that to avoid the ValueError bcrypt 5.x raises on longer inputs. Existing
+# $2b$ hashes (created via passlib's bcrypt backend) verify unchanged.
+_BCRYPT_MAX_BYTES = 72
+
+
+def _to_bytes(password: str) -> bytes:
+    return password.encode("utf-8")[:_BCRYPT_MAX_BYTES]
+
 
 def verify_password(plain: str, hashed: str) -> bool:
-    return pwd_context.verify(plain, hashed)
+    try:
+        return bcrypt.checkpw(_to_bytes(plain), hashed.encode("utf-8"))
+    except (ValueError, TypeError):
+        return False
 
 
 def hash_password(password: str) -> str:
-    return pwd_context.hash(password)
+    return bcrypt.hashpw(_to_bytes(password), bcrypt.gensalt()).decode("utf-8")
 
 
 def create_access_token(data: dict, expires_delta: Optional[timedelta] = None) -> str: