core: migrate from custom JSON storage to standard Drizzle migrations to improve database reliability and performance

This replaces the previous manual JSON file system with standard Drizzle migrations, enabling:
- Proper database schema migrations with timestamp-based versioning
- Batched migration for faster migration of large datasets
- Better data integrity with proper table schemas instead of JSON blobs
- Easier database upgrades and rollback capabilities

Migration changes:
- Todo table now uses individual columns with composite PK instead of JSON blob
- Share table removes unused download share data
- Session diff table moved from database table to file storage
- All migrations now use proper Drizzle format with per-folder layout

Users will see a one-time migration on next run that migrates existing JSON data to the new SQLite database.

package.json

@@ -38,6 +38,8 @@
       "@tailwindcss/vite": "4.1.11",
       "diff": "8.0.2",
       "dompurify": "3.3.1",
+      "drizzle-kit": "1.0.0-beta.12-a5629fb",
+      "drizzle-orm": "1.0.0-beta.12-a5629fb",
       "ai": "5.0.119",
       "hono": "4.10.7",
       "hono-openapi": "1.1.2",