feat: Add UserMapping constraints and schema improvements

## Database Schema Enhancements

### Migration 19 - Fixed
- Removed invalid composite index on session(visitor_id, user_id)
- visitor_id column doesn't exist in session table
- Fixed migration to only create valid indexes

### Migration 20 - New Constraints (with rollback support)
- Added websiteId foreign key to UserMapping table
- Added foreign key constraint to Website table for referential integrity
- Added composite index on UserMapping(websiteId, userId) for query performance
- Added composite index on Session(websiteId, userId) for join optimization

### Schema Updates
- Updated UserMapping model with websiteId field
- Added foreign key relationship: UserMapping -> Website
- Added userMappings relation to Website model
- Added composite indexes for better query performance

## Files Changed

### Modified
- prisma/schema.prisma - Added foreign keys and composite indexes
- prisma/migrations/19_add_user_id_mapping/migration.sql - Fixed invalid index

### New
- prisma/migrations/20_add_user_mapping_constraints/migration.sql - Forward migration
- prisma/migrations/20_add_user_mapping_constraints/rollback.sql - Rollback script

## Benefits
- ✅ Referential integrity enforced at database level
- ✅ Optimized queries for Umami sync operations
- ✅ Better join performance on Session table
- ✅ Rollback capability for safe schema changes

## Breaking Changes
- UserMapping table now requires websiteId field
- Existing data will be migrated to use first available website_id

## Migration Notes
Run the migration with:
```bash
npx prisma migrate deploy
```

To rollback if needed:
```bash
psql -d umami -f prisma/migrations/20_add_user_mapping_constraints/rollback.sql
```

prisma/migrations/20_add_user_mapping_constraints/rollback.sql

@@ -0,0 +1,16 @@
+-- Rollback migration for user_mapping constraints
+-- Run this script to revert the changes made in migration.sql
+
+-- Drop composite index on session table
+DROP INDEX IF EXISTS session_website_id_user_id_idx;
+
+-- Drop composite index on user_mapping table
+DROP INDEX IF EXISTS user_mapping_website_id_user_id_idx;
+
+-- Drop foreign key constraint
+ALTER TABLE user_mapping 
+DROP CONSTRAINT IF EXISTS user_mapping_website_id_fkey;
+
+-- Drop website_id column
+ALTER TABLE user_mapping DROP COLUMN IF EXISTS website_id;
+