Add sync scripts and readiness API with UI warnings

[pstaylor-patrick]

Related to #7

👋 TL;DR

Added database sync scripts to populate local, Neon, and Supabase databases from GCP source, plus a readiness API and UI indicators to show when platforms are empty or out of sync.

🔎 Details

This PR introduces a complete sync system for database targets and visual readiness indicators:

1. New sync scripts (scripts/db-sync.ts):

   • npm run db:sync:local, :neon, :supabase, :all to reset and repopulate targets from GCP dumps
   • Requires DB_SYNC_ALLOW_DESTRUCTIVE=true or interactive confirmation for safety
   • Verifies sync success with table and row counts

2. Readiness API (GET /api/readiness):

   • Returns table count and sample row count for each configured platform
   • Sets ready: true when both counts are > 0
   • Gracefully handles unreachable platforms

3. UI integration:

   • Platform pills show amber warning dots when not ready
   • Warning cards appear for selected empty platforms with sync instructions
   • DataDiff and PerformanceChart show diagnostic hints when row counts differ or are zero

4. Documentation:

   • Added .context/sync-and-readiness.md with full sync workflow and API docs
   • Updated CLAUDE.md with new npm scripts

5. Testing & robustness:

   • Added comprehensive unit tests for schema compare API
   • Improved error handling in schema compare route
   • Added safety checks to prevent syncing to GCP (read-only source)

✅ How to Test

1. Sync a target database:

   • Run npm run db:pull:dump to export from GCP first
   • Run npm run db:sync:local (confirm destruction when prompted)
   • Verify output shows table counts and row counts

2. Check readiness API:

   • Visit http://localhost:3002/api/readiness
   • Should return JSON array with platform statuses
   • Empty platforms should show ready: false

3. UI indicators:

   • Select an empty platform in the dashboard
   • Should see warning card with sync instructions
   • Platform pill should have amber warning dot

4. Data comparison:

   • Compare data between a ready and not-ready platform
   • Should see diagnostic hint about 0 rows
   • Performance chart should show warning about misleading latency

5. Schema compare error handling:

   • Test schema compare with a misconfigured platform
   • Should return 500 with error message instead of crashing

🥜 GIF