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.

docker-compose.yml

@@ -4,6 +4,7 @@ x-restart: &restart
 services:
   postgres:
     image: postgres:16-alpine
+    profiles: [db]
     <<: *restart
     env_file: .env
     volumes:
@@ -21,6 +22,7 @@ services:
     build:
       context: .
       dockerfile: obsidian.Dockerfile
+    profiles: [obsidian]
     <<: *restart
     volumes:
       - vault:/vault
@@ -31,6 +33,7 @@ services:
     container_name: beaver-gateway
     build:
       context: https://git.kotikot.com/beaver/beaver-gateway.git#${GATEWAY_REF:-main}
+    profiles: [gateway]
     <<: *restart
     depends_on:
       postgres: