Feat: Implement JWT bearer token authentication

Protects image upload, delete, and tag-update endpoints behind
Bearer token auth. Public read endpoints remain open. Angular SPA
gains a login page, auth interceptor, and route guard for /upload.

- JWTAuthProvider (HS256, sub/iat/exp, secrets.compare_digest)
- POST /api/v1/auth/token login endpoint
- require_auth FastAPI dependency on all write routes
- AuthService, LoginComponent, authInterceptor, authGuard
- Detail page hides write controls for unauthenticated visitors
- 43 unit tests passing; integration tests require Docker stack

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -13,3 +13,9 @@ API_BASE_URL=http://localhost:8000
 
 # Upload size limit in bytes (default 50 MiB)
 MAX_UPLOAD_BYTES=52428800
+
+# Owner credentials and JWT signing secret
+JWT_SECRET_KEY=change-me-to-a-long-random-string
+JWT_EXPIRY_SECONDS=86400
+OWNER_USERNAME=owner
+OWNER_PASSWORD=change-me