Build Summary Generation Service Core

Design this as a pure orchestrator — it contains no business logic itself, only the sequence of calls to injected services. Use the Facade pattern: `SummaryGenerationService` is the single public entry point; all dependencies are injected via constructor and typed as interfaces/abstract classes for mockability. Expose progress as a `Stream` using a `StreamController` so the BLoC layer can map progress events to UI states without polling. For idempotency, perform an upsert (Supabase `upsert` with `onConflict`) on `summary_periods` keyed on `(organisation_id, period_type, period_year, period_half_or_quarter)` — this is atomic and avoids a read-before-write race.

Notifications must be triggered only after the upsert confirms success (check Supabase response status). Structure `SummaryGenerationResult` as a sealed class with `SummaryGenerationSuccess` and `SummaryGenerationFailure` subtypes to force callers to handle both cases explicitly. The `warnings` field on the success type carries non-fatal issues (e.g., notification partial failures, missing thresholds for outlier detection). This service is the integration point for the 'Spotify Wrapped'-style gamification feature described in the workshop notes — design the `SummaryPeriod` data model to be extensible for per-peer-mentor summary records in a future phase.

Unit tests with flutter_test using fully mocked dependencies (ActivityAggregationRepository, PeriodCalculatorService, OutlierDetectionService, PushNotificationDispatcher, SummaryPeriodRepository): verify correct orchestration sequence; verify delta computation (positive, negative, zero-delta cases); verify idempotency (second call with same org/period updates rather than inserts); verify that notification failure does not roll back persistence; verify progress events are emitted in correct order; verify OrgIsolationException is thrown before any mock is called when orgId is invalid. Integration tests: run full generation against a Supabase test project with seeded data and assert `summary_periods` record is created with correct metrics and that the outlier report is linked. End-to-end test via TestFlight: trigger a generation run from the coordinator UI and verify the push notification arrives and deep-links to the correct summary screen. Minimum 90% branch coverage on the orchestration logic.