feat(deploy): split compose into prod base + dev override

Restructure the container setup to support registry-based deployment:
the default docker-compose.yml now pulls a prebuilt app image from the
Gitea container registry instead of building locally, while a new
docker-compose.dev.yml override preserves the local build + integration
test workflow for development.

Why:
- Production and self-hosting should consume a published image, not
  rebuild from source on the host. The default `docker compose up` now
  pulls the app image (git.linuxhg.com/bookhoard/bookhoard) alongside the
  public postgres image, with no build step required.
- Development still needs to build from source and run integration
  tests, so those concerns move to an override file the Makefile applies.
  Shared config (env, volumes, ports, healthchecks) lives in one place to
  avoid drift between environments.

Changes:
- docker-compose.yml (prod base): the app service now references
  `image: git.linuxhg.com/bookhoard/bookhoard:${IMAGE_TAG:-latest}` instead
  of a build context. The tests service is removed (moved to the
  override). IMAGE_TAG lets deployers pin or roll back a specific version.
- docker-compose.dev.yml (new override): adds the local `build:` context
  for the app and defines the integration `tests` service (profile-gated).
  Everything else is inherited from the base file via compose merging.
- Makefile: introduce a COMPOSE variable that merges the base and
  override (`-f docker-compose.yml -f docker-compose.dev.yml`); all dev
  targets now use it. Plain `docker compose` against the base file only
  remains the production path.
- README: quickstart updated to pull and start prebuilt images; clone URL
  points at the Gitea instance.

The development workflow (`make rebuild-app`, `make test-integration`,
etc.) is functionally unchanged.

docker-compose.dev.yml

@@ -0,0 +1,57 @@
+# Development override — merged on top of docker-compose.yml (the base/prod file).
+# Activated by all `make` targets via:
+#   COMPOSE = <runtime> compose -f docker-compose.yml -f docker-compose.dev.yml
+#
+# What this adds over prod:
+#   - Local image BUILDING (prod pulls a prebuilt image from the registry)
+#   - The integration-tests service (dev only, gated behind the "tests" profile)
+# Everything else (env vars, volumes, ports, healthchecks) is inherited from the base file.
+services:
+  # Build the app image locally instead of pulling from the registry
+  app:
+    build:
+      context: .
+      dockerfile: ./Dockerfile
+
+  # Integration Tests - runs against containerized app and db (dev only)
+  tests:
+    build:
+      context: .
+      dockerfile: ./Dockerfile
+      target: test-runner
+    container_name: bookhoard_tests
+    environment:
+      # Database Configuration
+      DATABASE_HOST: db
+      DATABASE_PORT: "5432"
+      DATABASE_USER: postgres
+      DATABASE_PASSWORD: ${DBPASS}
+      DATABASE_NAME: bookhoard
+      COOKIE_SECURE: false
+
+      # Application Configuration
+      JWT_SECRET: ${JWT_SECRET}
+      SERVER_PORT: "8765"
+
+      # Test Configuration
+      TEST_MODE: "true"
+      RATE_LIMIT_ENABLED: "false"
+      REQUESTS_PER_MINUTE: 1000
+
+      # Conversion Service Configuration
+      BOOKHOARD_CONVERSION_CACHE_DIR: /app/cache/kepub
+      BOOKHOARD_CONVERSION_TOOL: /usr/bin/kepubify
+      BOOKHOARD_CONVERSION_CACHE_TTL: 24h
+
+      # Test upload path (inside container)
+      TEST_UPLOAD_PATH: /app/uploads
+    depends_on:
+      db:
+        condition: service_healthy
+      app:
+        condition: service_healthy
+    volumes:
+      - ./uploads:/app/uploads
+      - bookhoard_conversion_cache:/app/cache/kepub
+    profiles:
+      - tests