feat(config): add optional IMMICH_DB_URL for pgvector spike

Optional Postgres DSN field on Config (REST creds stay required), env.example
entry, and a throwaway scripts/requirements-spike.txt (psycopg3 + pgvector)
kept out of the app's runtime deps. Unblocks the M1.5 pgvector feasibility spike.

apps/trip-cluster/app/config.py

@@ -17,6 +17,10 @@ class Config:
     immich_api_key: str
     anthropic_api_key: str
     data_dir: str
+    # Postgres DSN for read-only access to Immich's pgvector embeddings.
+    # Optional: only the pgvector spike / M1.5 visual-similarity work needs it;
+    # REST creds (immich_url/api_key) stay required.
+    immich_db_url: Optional[str] = None
 
     @property
     def db_path(self) -> str:
@@ -33,9 +37,11 @@ def load_config(env: Optional[Mapping] = None) -> Config:
     if missing:
         raise ConfigError(missing)
     data_dir = (env.get("DATA_DIR") or "").strip() or os.path.join(os.getcwd(), "data")
+    immich_db_url = (env.get("IMMICH_DB_URL") or "").strip() or None
     return Config(
         immich_url=env["IMMICH_URL"].strip().rstrip("/"),
         immich_api_key=env["IMMICH_API_KEY"].strip(),
         anthropic_api_key=(env.get("ANTHROPIC_API_KEY") or "").strip(),
         data_dir=data_dir,
+        immich_db_url=immich_db_url,
     )