Implement ActivityAttachmentRepository insert and query

Follow the existing repository pattern in the codebase (e.g., `ActivityRepository`). Inject `SupabaseClient` via constructor for testability. Extract `org_id` using `supabase.auth.currentSession?.user.userMetadata?['org_id']` or the equivalent JWT claim accessor used elsewhere in the project — check existing repositories for the canonical pattern. For `countAttachmentsForActivity`, use `.count(CountOption.exact)` on the Supabase query builder rather than fetching rows and calling `.length`.

For `insertAttachment`, call `.insert(attachment.toJson()).select().single()` to get the server-persisted record back. Define `ActivityAttachment.fromJson` to handle nullable `deleted_at` gracefully. Place the class in `lib/data/repositories/activity_attachment_repository.dart` and register it as a Riverpod `Provider` or `StateNotifierProvider` following the project's DI convention.

Unit tests using `flutter_test` with a mocked `SupabaseClient` (using `mocktail` or `mockito`). Test cases: (1) `insertAttachment` — mock returns inserted row JSON, assert returned model matches; (2) `insertAttachment` — mock throws PostgrestException, assert typed exception is rethrown; (3) `getActiveAttachmentsForActivity` — mock returns list JSON, assert returned list length and field mapping; (4) `getActiveAttachmentsForActivity` — empty result, assert empty list returned (not null or exception); (5) `countAttachmentsForActivity` — mock returns count response, assert correct integer; (6) all methods — verify org_id filter is applied by inspecting the query builder calls via mock verification. Target 100% branch coverage on the repository class.