feat: add CRUD operations for external_releases table

Implement GetExternalRelease, SaveExternalRelease,
GetExternalReleasesByArtist, GetIgnoredReleases, and
SetReleaseIgnored with 12 table-driven tests covering
success and error cases.

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

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

@@ -97,14 +97,14 @@ Build the foundation layer of NaviWatcher: a greenfield Go project with zero exi
 - [x] run tests — must pass before task 5
 
 ### Task 5: Database layer — CRUD operations for external_releases
-- [ ] implement `GetExternalRelease(db *DB, rgid string) (*ExternalRelease, error)`
-- [ ] implement `SaveExternalRelease(db *DB, release *ExternalRelease) error`
-- [ ] implement `GetExternalReleasesByArtist(db *DB, artistID string) ([]ExternalRelease, error)`
-- [ ] implement `GetIgnoredReleases(db *DB) ([]ExternalRelease, error)`
-- [ ] implement `SetReleaseIgnored(db *DB, rgid string, ignored bool) error`
-- [ ] write tests for all CRUD operations (success and error cases)
-- [ ] write tests for cache TTL logic (if implemented at this layer)
-- [ ] run tests — must pass before task 6
+- [x] implement `GetExternalRelease(db *DB, rgid string) (*ExternalRelease, error)`
+- [x] implement `SaveExternalRelease(db *DB, release *ExternalRelease) error`
+- [x] implement `GetExternalReleasesByArtist(db *DB, artistID string) ([]ExternalRelease, error)`
+- [x] implement `GetIgnoredReleases(db *DB) ([]ExternalRelease, error)`
+- [x] implement `SetReleaseIgnored(db *DB, rgid string, ignored bool) error`
+- [x] write tests for all CRUD operations (success and error cases)
+- [x] write tests for cache TTL logic (if implemented at this layer)
+- [x] run tests — must pass before task 6
 
 ### Task 6: Database layer — CRUD operations for notifications_sent
 - [ ] implement `MarkNotificationSent(db *DB, rgid string) error`