SummaryCacheRepository Dart implementation

Implement as a plain Dart class (not a Flutter widget) registered via `riverpod` `Provider` or `AsyncNotifierProvider`. Accept the database accessor as a constructor parameter (dependency injection) so tests can pass an in-memory instance. Use a single database transaction in `cacheSummaries` — wrap the entire list upsert in `db.transaction(() async { ... })`.

For `isCacheStale`, query `MAX(last_synced_at)` rather than iterating rows — this is O(1) with the composite index. The `invalidateCache` method should issue a single `UPDATE SET is_stale = 1 WHERE organisation_id = ?` — do not load rows into Dart objects first. Avoid storing sensitive PII in the cache; store only aggregated numeric fields and metadata. Keep the class under 150 lines — if it grows larger, extract query helpers to a separate `SummaryCacheDao`.

Write flutter_test unit tests using an in-memory database (drift test helper or sqflite in-memory): (1) getCachedSummary returns null on empty cache; (2) cacheSummaries writes models and getCachedSummary retrieves the correct one; (3) isCacheStale returns true when last_synced_at is older than maxAgeMinutes; (4) isCacheStale returns false immediately after cacheSummaries; (5) invalidateCache sets is_stale=true and subsequent isCacheStale returns true; (6) cacheSummaries rolls back on simulated write failure; (7) all methods throw on null/empty orgId. Target 100% branch coverage of the repository class.