fix: resolve Docker database connection issues - COMPLETE

Major fixes to database connectivity in Docker:

1. Fix alembic.ini - Changed localhost to postgres (line 90)
   - This was the root cause of localhost connection errors

2. Fix database.py default - Changed localhost to postgres
   - Ensures correct default when env var not set

3. Fix config.py default - Changed localhost to postgres
   - Consistent configuration across all files

4. Fix .env file - Changed DATABASE_URL from localhost to postgres
   - Prevents local dev config from overriding Docker config

5. Update Dockerfile.backend - Add debug logging to verify env vars

6. Fix docker-compose.yml frontend port - Changed 3000 to 8888

7. Fix Celery commands - Use 'uv run celery' instead of just 'celery'

8. Remove obsolete 'version' attribute from docker-compose.yml

Verification:
- DATABASE_URL env var: postgresql+asyncpg://postgres:postgres@postgres:5432/mockupaws ✅
- Backend now connects to postgres:5432 instead of localhost ✅
- Frontend accessible at http://localhost:8888 ✅

Note: There's a separate migration error with index creation
(idx_logs_recent using NOW() - requires IMMUTABLE function).
This is a database migration issue, not a connection issue.

src/core/database.py

@@ -4,11 +4,14 @@ import os
 from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
 from sqlalchemy.orm import declarative_base
 
-# URL dal environment o default per dev
+# URL dal environment o default per Docker
 DATABASE_URL = os.getenv(
-    "DATABASE_URL", "postgresql+asyncpg://postgres:postgres@localhost:5432/mockupaws"
+    "DATABASE_URL", "postgresql+asyncpg://postgres:postgres@postgres:5432/mockupaws"
 )
 
+# Debug: stampa la DATABASE_URL all'avvio
+print(f"DEBUG - DATABASE_URL: {DATABASE_URL}", flush=True)
+
 # Engine async
 engine = create_async_engine(
     DATABASE_URL,