fix: add alembic files to development Docker image and fix database configuration

- Added COPY commands for alembic.ini and alembic/ directory in development Dockerfile
- Updated alembic.ini to use correct database credentials for Docker environment
- Fixed database URL to point to 'db' host instead of 'localhost'
- Updated password to match docker-compose environment variables

These changes resolve the database startup issues where:
1. Alembic migrations couldn't run because files were missing from container
2. Database connection failed due to incorrect credentials
3. API couldn't start because required tables didn't exist

Now the full development environment starts properly with:
- Database migrations running automatically
- API connecting to database successfully
- UI accessible on port 3000
- API accessible on port 8000

api/alembic.ini

@@ -1,7 +1,7 @@
 [alembic]
 script_location = alembic
 prepend_sys_path = .
-sqlalchemy.url = postgresql+asyncpg://rh_user:change_me@localhost:5432/rehearsalhub
+sqlalchemy.url = postgresql+asyncpg://rh_user:changeme_password_123@db:5432/rehearsalhub
 
 [loggers]
 keys = root,sqlalchemy,alembic