Implement RecognitionTierRepository

This is the simplest of the three repositories. Follow the exact same pattern as BadgeRepository for consistency — same exception hierarchy, same interface pattern, same Riverpod provider registration. The `colourToken` field should be a validated string matching the project's design token naming convention (e.g., 'tier_bronze', 'tier_silver', 'tier_gold') — consider defining a `TierColourToken` enum or constant class rather than accepting arbitrary strings. For `updateTier`, construct the update map by only including non-null fields from `UpdateTierParams` to avoid overwriting fields the caller did not intend to change.

Use a `toUpdateJson()` method on `UpdateTierParams` that returns `Map` with only non-null entries. Since threshold ordering is critical for tier resolution logic (awarding the correct tier based on assignment count), add a comment in `fetchAllTiers` explaining why server-side ordering is mandatory here.

Unit tests with mocked SupabaseClient: (1) fetchAllTiers returns list sorted by threshold ascending from mock data, (2) fetchAllTiers returns empty list when Supabase returns empty array, (3) fetchTierById returns null when not found, (4) createTier with threshold < 0 throws InvalidTierException before calling Supabase, (5) updateTier sends only non-null fields to Supabase update, (6) Supabase PostgrestException is wrapped in TierRepositoryException. Integration test verifying create → fetch → update cycle against local Supabase. Minimum 85% line coverage.