Implement Bufdir alignment validation checks

The independence of the two queries is the entire point of this validator — if both queries use the same materialized view, drift will never be detected. The RPC function (`rpc_bufdir_export_totals`) must query the `activities` base table directly, replicating the aggregation logic from the Bufdir export pipeline, not the materialized view. This should be coordinated with task-004. Percentage drift formula: `percentageDrift = (abs(viewTotal - rpcTotal) / max(viewTotal, 1)) * 100`.

Use `max(viewTotal, 1)` as denominator to avoid division by zero while still reporting meaningful drift when viewTotal is 0 but rpcTotal is non-zero. The 5-minute rate limit should be implemented as a simple in-memory `DateTime? lastValidationRun` field on the service — no persistence needed since drift is a transient concern. Register the validator as a Riverpod provider and trigger it from the Stats BLoC's data-loading state transition.

Ensure `analyticsService.logWarning` calls include `chapter_id` (not user ID) and the reporting period for traceability without PII exposure.

Unit tests with mocked `StatsRepository` and mocked Supabase RPC client: (1) aligned case — both sources return same totals, `isAligned = true`, no analytics warning logged; (2) drift within threshold (0.005%) — `isAligned = true`, no warning; (3) drift above threshold (0.02%) — `isAligned = false`, `analyticsService.logWarning` called with correct field name and drift value; (4) RPC failure — `isAligned = false`, `analyticsService.logError` called, no exception propagated to caller; (5) zero-value stats — `percentageDrift = 0.0`, no division-by-zero; (6) feature flag disabled — validator returns immediately with `isAligned = true` without calling either data source. Integration test: seed known totals in local Supabase, run validator, assert `isAligned = true`.