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/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