feat: add CRUD operations for notifications_sent table

Implement MarkNotificationSent, IsNotificationSent, and
GetUnnotifiedReleases with LEFT JOIN query. Includes 10
table-driven tests covering success, error, idempotent,
and edge cases. Also go fmt cleanup on existing files.

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

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

@@ -107,12 +107,12 @@ Build the foundation layer of NaviWatcher: a greenfield Go project with zero exi
 - [x] run tests — must pass before task 6
 
 ### Task 6: Database layer — CRUD operations for notifications_sent
-- [ ] implement `MarkNotificationSent(db *DB, rgid string) error`
-- [ ] implement `IsNotificationSent(db *DB, rgid string) (bool, error)`
-- [ ] implement `GetUnnotifiedReleases(db *DB) ([]ExternalRelease, error)` — joins external_releases with notifications_sent to find unsent
-- [ ] write tests for MarkNotificationSent and IsNotificationSent
-- [ ] write tests for GetUnnotifiedReleases (with and without existing notifications)
-- [ ] run tests — must pass before task 7
+- [x] implement `MarkNotificationSent(db *DB, rgid string) error`
+- [x] implement `IsNotificationSent(db *DB, rgid string) (bool, error)`
+- [x] implement `GetUnnotifiedReleases(db *DB) ([]ExternalRelease, error)` — joins external_releases with notifications_sent to find unsent
+- [x] write tests for MarkNotificationSent and IsNotificationSent
+- [x] write tests for GetUnnotifiedReleases (with and without existing notifications)
+- [x] run tests — must pass before task 7
 
 ### Task 7: Docker setup
 - [ ] create `Dockerfile` with multi-stage build: build stage (golang:1.21-alpine) + runtime stage (alpine:latest)