Normalize ranking persistence and migrate existing stores

[hubab1]

Summary

• Introduce the V6 normalized ranking schema for crawls, ranking facts, deduplicated app revisions, tracked-history links, and resumable migration state.
• Refactor ranking refresh, history, keyword tables, research, insights, suggestions, and MCP reads to query normalized facts directly.
• Preserve existing tracked history with recovery crawls when a tracked result differs from its canonical crawl.
• Stop producing redundant AppKeywordStats ranking data while preserving all existing rows.

Why

Ranking pages and app metadata were persisted repeatedly under canonical crawls and tracked daily snapshots. That duplication grew a real production store beyond 2 GB and made relationship loading and mutation unnecessarily expensive.

The migration removes duplicate storage without reducing the amount of tracked information. Legacy ranking tables are deleted only after exact fact-by-fact validation succeeds.

Migration safety

• Uses a lightweight SwiftData V5 → V6 schema migration followed by a resumable, batched application migration.
• Uses stable numeric offsets so mixed ASCII, accented, and CJK observation keys migrate consistently.
• Preserves all 1,373,792 canonical ranking facts and 7,825 tracked daily snapshots from the production-copy fixture.
• Adds 1,457 recovery crawls containing 7,533 facts where tracked history differs from canonical data.
• Preserves all 177,604 existing AppKeywordStats rows.
• Deletes 1,099,177 redundant tracked-result rows only after exact validation.

Measured impact

The production database used for validation contained 7,491 crawls and 1,373,792 canonical ranking facts.

• Logical SQLite usage: 2.17 GB → 1.30 GB (40.1% reduction).
• Migration trial 1: 17m 24s, 348 MiB peak RSS.
• Migration trial 2: 18m 11s, 351 MiB peak RSS.
• Both trials started from a freshly copied, closed production store and produced identical final counts.
• The original production store remained unmigrated and unchanged.

Validation

• 670/670 tests passed across 58 suites.
• Added migration coverage for resumability, rollback/validation behavior, tracked recovery crawls, exact counts, and mixed-Unicode multi-batch ordering.
• PRAGMA quick_check: ok after both production-copy migrations.
• Rebuilt app launched against the migrated store and rendered the real 338-keyword tracked-app table, rankings, movement, distribution, and trend summaries.
• Clean idle CPU measured at 0%.
• git diff --check passed.