fix: rewrite migration 0009 for SQLite compatibility and add migration validation (#230)

SQLite rejects ALTER TABLE ADD COLUMN with expression defaults like
DEFAULT (unixepoch()), which Drizzle-kit generated for the imported_at
column. This broke upgrades from v3.12.x to v3.13.0 (#228, #229).

Changes:
- Rewrite migration 0009 using table-recreation pattern (CREATE, INSERT
  SELECT, DROP, RENAME) instead of ALTER TABLE
- Add migration validation script with SQLite-specific lint rules that
  catch known invalid patterns before they ship
- Add upgrade-path testing with seeded data and verification fixtures
- Add runtime repair for users whose migration record may be stale
- Add explicit migration validation step to CI workflow

Fixes #228
Fixes #229

.github/workflows/astro-build-test.yml

@@ -48,6 +48,12 @@ jobs:
 
       - name: Run tests
         run: bun test --coverage
+      
+      - name: Check Drizzle migrations
+        run: bun run db:check
+
+      - name: Validate migrations (SQLite lint + upgrade path)
+        run: bun test:migrations
 
       - name: Build Astro project
         run: bunx --bun astro build