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/schema.prisma

@@ -43,6 +43,7 @@ model Session {
   region     String?   @db.VarChar(20)
   city       String?   @db.VarChar(50)
   distinctId String?   @map("distinct_id") @db.VarChar(50)
+  userId     String?   @map("user_id") @db.VarChar(36)
   createdAt  DateTime? @default(now()) @map("created_at") @db.Timestamptz(6)
 
   websiteEvents WebsiteEvent[]
@@ -52,6 +53,8 @@ model Session {
   @@index([createdAt])
   @@index([websiteId])
   @@index([websiteId, createdAt])
+  @@index([userId])
+  @@index([websiteId, userId])
   @@index([websiteId, createdAt, browser])
   @@index([websiteId, createdAt, os])
   @@index([websiteId, createdAt, device])
@@ -76,14 +79,15 @@ model Website {
   updatedAt DateTime? @updatedAt @map("updated_at") @db.Timestamptz(6)
   deletedAt DateTime? @map("deleted_at") @db.Timestamptz(6)
 
-  user        User?         @relation("user", fields: [userId], references: [id])
-  createUser  User?         @relation("createUser", fields: [createdBy], references: [id])
-  team        Team?         @relation(fields: [teamId], references: [id])
-  eventData   EventData[]
-  reports     Report[]
-  revenue     Revenue[]
-  segments    Segment[]
-  sessionData SessionData[]
+  user         User?          @relation("user", fields: [userId], references: [id])
+  createUser   User?          @relation("createUser", fields: [createdBy], references: [id])
+  team         Team?          @relation(fields: [teamId], references: [id])
+  eventData    EventData[]
+  reports      Report[]
+  revenue      Revenue[]
+  segments     Segment[]
+  sessionData  SessionData[]
+  userMappings UserMapping[]
 
   @@index([userId])
   @@index([teamId])
@@ -330,3 +334,22 @@ model Pixel {
   @@index([createdAt])
   @@map("pixel")
 }
+
+model UserMapping {
+  id           Int       @id @default(autoincrement())
+  websiteId    String    @map("website_id") @db.Uuid
+  visitorId    String    @map("visitor_id") @db.VarChar(36)
+  userId       String    @map("user_id") @db.VarChar(36)
+  firstSeenAt  DateTime  @default(now()) @map("first_seen_at") @db.Timestamptz(6)
+  lastSeenAt   DateTime  @default(now()) @map("last_seen_at") @db.Timestamptz(6)
+  sessionCount Int       @default(1) @map("session_count")
+
+  website Website @relation(fields: [websiteId], references: [id])
+
+  @@unique([visitorId, userId])
+  @@index([visitorId])
+  @@index([userId])
+  @@index([lastSeenAt])
+  @@index([websiteId, userId])
+  @@map("user_mapping")
+}