feat(db): idempotent migrations + auto-deploy on startup

- Rewrite 0000_init with IF NOT EXISTS on all tables, indexes and enums;
  wrap FK constraints in DO/EXCEPTION blocks so re-runs never fail
- Add missing suggestions table to init migration
- Add migration 0002 to create message_logs table (IF NOT EXISTS)
- Add entrypoint.sh: runs prisma migrate deploy before starting the server
- Update Dockerfile to copy entrypoint.sh and use it as CMD
- Add prisma:migrate script to package.json for manual runs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/prisma/schema.prisma

@@ -194,10 +194,10 @@ model suggestions {
 
 model message_logs {
   id         String   @id @default(dbgenerated("uuid_generate_v4()")) @db.Uuid
-  channel    String   @db.VarChar(10)  // 'sms' | 'email'
+  channel    String   @db.VarChar(10)
   recipient  String   @db.VarChar(255)
   body       String
-  status     String   @db.VarChar(20)  // 'sent' | 'error'
+  status     String   @db.VarChar(20)
   error      String?
   created_at DateTime @default(now()) @db.Timestamptz(6)
 }