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.

config.yaml.example

@@ -0,0 +1,26 @@
+server:
+  host: "0.0.0.0"
+  port: 8080
+  # Basic Auth for Web UI access
+  username: "admin"
+  password: "CHANGE_ME"
+
+navidrome:
+  url: "http://localhost:4533"
+  user: "watcher_service"
+  password: "CHANGE_ME"
+
+musicbrainz:
+  user_agent: "NaviWatcher/1.0 ( your-email@example.com )"
+  cache_ttl: 24h
+
+telegram:
+  enabled: true
+  token: "bot_token"
+  chat_id: "your_chat_id"
+  cron_schedule: "0 10 * * *"
+
+scanner:
+  fuzzy_threshold: 0.85
+  ignore_bootlegs: true
+  include_compilations: true