feat(infra): compose profiles, Makefile, local override example

All services sit behind profiles (db, gateway, obsidian) selected via
COMPOSE_PROFILES in .env; Makefile with the usual verbs and a `deploy`
target that pushes main:stable.

.gitignore

@@ -3,3 +3,4 @@ config.json
 .venv/
 __pycache__/
 .idea
+docker-compose.override.yml