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.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docker-compose.override.yml.example

@@ -0,0 +1,5 @@
+# Local overrides, not committed. Copy to docker-compose.override.yml.
+services:
+  postgres:
+    ports:
+      - "127.0.0.1:5432:5432"