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>

frontend/next.config.ts

@@ -5,7 +5,11 @@ const nextConfig: NextConfig = {
     return [
       {
         source: "/api/:path*",
-        destination: `${process.env.NEXT_PUBLIC_API_URL || "http://localhost:8000"}/api/:path*`,
+        // Server-side proxy for direct :3000 access. Uses the internal service
+        // hostname inside Docker (API_INTERNAL_URL=http://backend:8000); falls
+        // back to localhost for non-container dev. Not used when served via
+        // nginx, which proxies /api itself.
+        destination: `${process.env.API_INTERNAL_URL || "http://localhost:8000"}/api/:path*`,
       },
     ];
   },

frontend/src/lib/api.ts

@@ -1,4 +1,8 @@
-const BASE = process.env.NEXT_PUBLIC_API_URL || "http://localhost:8000";
+// Empty string = same-origin. API calls go to /api/... on whatever host served
+// the page, and are proxied to the backend by nginx (port 80) or the Next.js
+// rewrite in next.config.ts (port 3000). This keeps everything same-origin so
+// it works via localhost, the LAN IP, and the Cloudflare tunnel with no CORS.
+const BASE = process.env.NEXT_PUBLIC_API_URL ?? "";
 
 let authToken: string | null = null;