feat: add database layer with schema migrations

Add internal/database package with SQLite3 connection management,
versioned migration system, and three schema tables (artist_settings,
external_releases, notifications_sent). Includes 7 tests covering
initialization, migration idempotency, Close() behavior, and schema
validation using in-memory SQLite.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-05-20-foundation-layer.md

@@ -75,16 +75,16 @@ Build the foundation layer of NaviWatcher: a greenfield Go project with zero exi
 - [x] run tests — must pass before task 3
 
 ### Task 3: Database layer — schema and migrations
-- [ ] create `internal/database/database.go` with DB struct and `New(dbPath string) (*DB, error)` constructor
-- [ ] implement schema migration system (versioned migrations table + ordered migration files or functions)
-- [ ] create migration 001: `artist_settings` table (id, name, ignore_singles, ignore_compilations, monitored)
-- [ ] create migration 002: `external_releases` table (rgid PK, artist_id, title, type, release_date, is_ignored)
-- [ ] create migration 003: `notifications_sent` table (rgid FK, sent_at)
-- [ ] implement `Close()` method with proper connection cleanup
-- [ ] write tests for database initialization and schema creation (use in-memory SQLite `:memory:`)
-- [ ] write tests for migration idempotency (running migrations twice should not fail)
-- [ ] write tests for Close() behavior
-- [ ] run tests — must pass before task 4
+- [x] create `internal/database/database.go` with DB struct and `New(dbPath string) (*DB, error)` constructor
+- [x] implement schema migration system (versioned migrations table + ordered migration files or functions)
+- [x] create migration 001: `artist_settings` table (id, name, ignore_singles, ignore_compilations, monitored)
+- [x] create migration 002: `external_releases` table (rgid PK, artist_id, title, type, release_date, is_ignored)
+- [x] create migration 003: `notifications_sent` table (rgid FK, sent_at)
+- [x] implement `Close()` method with proper connection cleanup
+- [x] write tests for database initialization and schema creation (use in-memory SQLite `:memory:`)
+- [x] write tests for migration idempotency (running migrations twice should not fail)
+- [x] write tests for Close() behavior
+- [x] run tests — must pass before task 4
 
 ### Task 4: Database layer — CRUD operations for artist_settings
 - [ ] implement `GetArtistSettings(db *DB, id string) (*ArtistSettings, error)`