chore: docker compose ergonomics — rename dev override, bake dev defaults

- Renamed docker-compose.dev.yml → docker-compose.override.yml so Docker
  Compose auto-merges it. `docker compose up` (no -f) now Just Works for
  local development.

- Removed the `env_file: .env` requirement from every service in the base
  file. Operators no longer need to create a .env to bring the stack up.

- Baked sane dev defaults directly into docker-compose.yml via
  ${VAR:-default} interpolation:
    DB_USER=fabledcurator
    DB_PASSWORD=fabledcurator_dev
    DB_NAME=fabledcurator
    SECRET_KEY=dev_secret_key_not_for_production_change_me
    LOG_LEVEL=INFO (overridden to DEBUG by the dev override)

  Defaults are insecure but explicitly named so. For production, override
  via shell env vars or a .env file at the project root.

- README quick-start simplified to a single `docker compose up -d`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.override.yml

@@ -0,0 +1,45 @@
+services:
+  postgres:
+    ports:
+      - "5432:5432"
+
+  redis:
+    ports:
+      - "6379:6379"
+
+  web:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    environment:
+      LOG_LEVEL: DEBUG
+    volumes:
+      - ./backend:/app/backend
+      - ./alembic:/app/alembic
+
+  worker:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    environment:
+      LOG_LEVEL: DEBUG
+    volumes:
+      - ./backend:/app/backend
+
+  scheduler:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    environment:
+      LOG_LEVEL: DEBUG
+    volumes:
+      - ./backend:/app/backend
+
+  ml-worker:
+    build:
+      context: .
+      dockerfile: Dockerfile.ml
+    environment:
+      LOG_LEVEL: DEBUG
+    volumes:
+      - ./backend:/app/backend