fix(rank-tracking): use dialect-native timestamp cutoffs for comparisons

[Ayush7614]

Summary

• Rank-tracking history and compare cutoffs always formatted timestamps like SQLite current_timestamp (YYYY-MM-DD HH:MM:SS).
• Hosted Postgres stores checkedAt as ISO text (YYYY-MM-DDTHH:MM:SS.MSZ). Lexicographic gte/lte across those formats breaks same-day windows ( < T), so 1d/7d/30d deltas and history can be wrong.
• Cutoffs now match the active database dialect.

How to review

1. rankTrackingTimestamps.ts — toRankTrackingCutoffTimestamp
2. Call sites in snapshotQueries.ts and rankTrackingResults.ts
3. Unit tests cover both D1 and Postgres formats

Test plan

• pnpm exec vitest run src/server/features/rank-tracking/repositories/snapshotQueries.test.ts
• On hosted Postgres, verify a 1d compare includes same-calendar-day snapshots written after the cutoff