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.

.env.example

@@ -1,5 +1,9 @@
 IMMICH_URL=http://your-immich-host:2283
 IMMICH_API_KEY=your-immich-api-key
+# Optional: Postgres DSN for read-only access to Immich's pgvector embeddings.
+# Only needed by the pgvector spike (scripts/pgvector_spike.py) / M1.5 visual similarity.
+# Format: postgresql://USER:PASSWORD@HOST:PORT/DBNAME  (Immich defaults: user=postgres, db=immich)
+# IMMICH_DB_URL=postgresql://postgres:your-db-password@your-immich-host:5432/immich
 ANTHROPIC_API_KEY=
 DATA_DIR=./data
 UID=1000