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>

api/app/auth/jwt_provider.py

@@ -0,0 +1,51 @@
+import secrets
+from datetime import UTC, datetime, timedelta
+
+import jwt
+from fastapi import HTTPException
+
+from app.auth.provider import AuthProvider, Identity
+
+_UNAUTHORIZED = HTTPException(
+    status_code=401, detail={"detail": "Unauthorized", "code": "unauthorized"}
+)
+
+
+class JWTAuthProvider(AuthProvider):
+    def __init__(
+        self,
+        secret_key: str,
+        expiry_seconds: int,
+        owner_username: str,
+        owner_password: str,
+    ) -> None:
+        self._secret_key = secret_key
+        self._expiry_seconds = expiry_seconds
+        self._owner_username = owner_username
+        self._owner_password = owner_password
+
+    def create_token(self) -> str:
+        now = datetime.now(tz=UTC)
+        payload = {
+            "sub": "owner",
+            "iat": now,
+            "exp": now + timedelta(seconds=self._expiry_seconds),
+        }
+        return jwt.encode(payload, self._secret_key, algorithm="HS256")
+
+    def verify_credentials(self, username: str, password: str) -> bool:
+        username_ok = secrets.compare_digest(username, self._owner_username)
+        password_ok = secrets.compare_digest(password, self._owner_password)
+        return username_ok and password_ok
+
+    async def get_identity(self, authorization: str | None) -> Identity:
+        if not authorization or not authorization.startswith("Bearer "):
+            raise _UNAUTHORIZED
+        token = authorization.removeprefix("Bearer ")
+        try:
+            jwt.decode(token, self._secret_key, algorithms=["HS256"])
+        except jwt.ExpiredSignatureError:
+            raise _UNAUTHORIZED from None
+        except jwt.InvalidTokenError:
+            raise _UNAUTHORIZED from None
+        return Identity(id="owner", anonymous=False)