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/main.py

@@ -22,9 +22,16 @@ app = FastAPI(
     lifespan=lifespan,
 )
 
+# The app is normally served same-origin (nginx proxies /api to the backend),
+# so CORS is not exercised in the primary flow. This allowlist exists for direct
+# browser access to :8000 during development and for any explicitly configured
+# origins. Extra origins can be added via the CORS_ORIGINS env var (comma-separated).
+_extra_origins = [o.strip() for o in (settings.CORS_ORIGINS or "").split(",") if o.strip()]
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=["http://localhost:3000", "http://frontend:3000"],
+    allow_origins=["http://localhost:3000", "http://frontend:3000", *_extra_origins],
+    # Also allow localhost and private-network hosts on any port (dev convenience).
+    allow_origin_regex=r"^https?://(localhost|127\.0\.0\.1|10\.\d+\.\d+\.\d+|192\.168\.\d+\.\d+|172\.(1[6-9]|2\d|3[01])\.\d+\.\d+)(:\d+)?$",
     allow_credentials=True,
     allow_methods=["*"],
     allow_headers=["*"],