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,
     )

apps/trip-cluster/tests/test_config.py

@@ -16,3 +16,13 @@ def test_anthropic_optional_and_paths(tmp_path):
     assert cfg.anthropic_api_key == ""           # optional in M1
     assert cfg.db_path == os.path.join(str(tmp_path), "trip-cluster.db")
     assert cfg.thumbs_dir == os.path.join(str(tmp_path), "thumbs")
+
+
+def test_immich_db_url_optional():
+    # DSN is optional — only the pgvector spike / M1.5 need it; REST creds stay required.
+    cfg = load_config({"IMMICH_URL": "http://x", "IMMICH_API_KEY": "k"})
+    assert cfg.immich_db_url is None
+
+    cfg = load_config({"IMMICH_URL": "http://x", "IMMICH_API_KEY": "k",
+                       "IMMICH_DB_URL": "  postgresql://u:p@h:5432/immich  "})
+    assert cfg.immich_db_url == "postgresql://u:p@h:5432/immich"  # trimmed