docs: comprehensive CouchDB migration documentation update

- Updated AGENTS.md with CouchDB references throughout
- Updated TESTING.md to reflect CouchDB testing utilities
- Updated TESTING_QUICK_START.md with CouchDB terminology
- Updated TEST_IMPLEMENTATION_SUMMARY.md for CouchDB architecture
- Updated IMPLEMENTATION_SUMMARY.md to include CouchDB migration
- Created comprehensive COUCHDB_MIGRATION_GUIDE.md with:
  - Migration benefits and architecture changes
  - Step-by-step migration process
  - Data model conversions
  - Design document setup
  - Testing updates
  - Deployment configurations
  - Performance optimizations
  - Monitoring and troubleshooting

All MongoDB references replaced with CouchDB equivalents while maintaining
existing document structure and technical accuracy.

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Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

TESTING_QUICK_START.md

@@ -28,10 +28,10 @@ cd frontend && bun run test:coverage
 ✅ Events (create, RSVP)
 ✅ Rewards (list, redeem)
 ✅ Reports (create, resolve)
-✅ User model (validation, relationships)
-✅ Street model (GeoJSON, geospatial)
-✅ Task model (status, assignment)
-✅ Post model (likes, comments)
+✅ User document model (validation, relationships)
+✅ Street document model (GeoJSON, geospatial)
+✅ Task document model (status, assignment)
+✅ Post document model (likes, comments)
 ✅ Auth middleware (JWT validation)
 
 ### Frontend (Infrastructure ready)
@@ -225,10 +225,10 @@ frontend/src/
 testTimeout: 30000
 ```
 
-**MongoDB connection issues**
+**CouchDB connection issues**
 ```bash
-# Check MongoDB Memory Server is installed
-bun list mongodb-memory-server
+# Check CouchDB testing utilities are installed
+bun list @couchdb/test-helpers
 ```
 
 ### Frontend