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.

README.md

@@ -25,7 +25,7 @@ A modern self-hosted media library system built with Go, PostgreSQL, HTMX, and T
 
 ```bash
 # 1. Clone the repository
-git clone https://github.com/yourusername/bookhoard.git
+git clone https://git.linuxhg.com/Bookhoard/bookhoard.git
 cd bookhoard
 
 # 2. Set up environment
@@ -35,8 +35,10 @@ cp .env.example .env
 #   DBPASS: openssl rand -hex 16
 # Edit .env with your generated values
 
-# 3. Start the server
-podman-compose up --build -d  # or: docker-compose up --build -d
+# 3. Pull images and start the server
+docker compose pull
+docker compose up -d
+# Optionally pin a specific version: set IMAGE_TAG in .env (defaults to "latest")
 
 # 4. Open your browser
 open http://localhost:8765