fix: move admin SQL to db layer for SQLite compat, remove schema page
Admin page failed on SQLite with "unrecognized token: ':''" due to
DuckDB-specific COUNT(*)::INTEGER and pragma_database_size() syntax.
Moved all inline SQL from admin routes into dialect-aware db.ts
functions. Removed unused schema endpoint and UI. Added resync
fallback via triggerAutoBackfill when onResync callback is not provided.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>