feat: add foundation layer (Go module, config, database, Docker)

Squashed commits from foundation-layer branch:

- Initialize Go module and project skeleton (cmd/naviwatcher/main.go)
- Add configuration management with YAML parsing and validation
- Add database layer with schema migrations (artist_settings, external_releases, notifications_sent)
- Add CRUD operations for artist_settings, external_releases, notifications_sent
- Add Docker setup with multi-stage build and docker-compose
- Verify acceptance criteria (tests, vet, fmt)
- Update README.md with build/run/test instructions
- Fix: filter ignored releases in GetUnnotifiedReleases (spec compliance)
- Fix: add FK constraint on notifications_sent.rgid
- Fix: add config.yaml to .gitignore (security)
- Fix: run Docker container as non-root user
- Fix: pin alpine:3.21 instead of alpine:latest
- Fix: wrap migrations in transactions for atomicity

All 49 tests pass, go vet clean, Docker image builds successfully.

docker-compose.yml

@@ -0,0 +1,11 @@
+services:
+  naviwatcher:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    container_name: naviwatcher
+    ports:
+      - "8080:8080"
+    volumes:
+      - ./data:/app/data
+    restart: unless-stopped