Migrate flight-blender from Django to FastAPI (partial)

[Copilot]

Partial migration of flight-blender from Django/DRF to FastAPI/SQLAlchemy. This PR establishes the core infrastructure and converts a subset of modules. The migration is incomplete — remaining view modules, app entry point, and cleanup are still needed.

What's done

• Dependencies (pyproject.toml): Replaced Django, DRF, django-celery-beat, dj-database-url, channels/channels-redis with FastAPI, SQLAlchemy 2.0, Alembic, websockets
• Core infrastructure:
  • flight_blender/config.py — env-based config replacing settings.py
  • flight_blender/db.py — SQLAlchemy engine, SessionLocal, Base, get_db() dependency
  • flight_blender/celery.py — decoupled from Django settings, explicit task discovery
• All 28 Django ORM models → SQLAlchemy across 8 models.py files
• common/database_operations.py (1361 lines) — full rewrite from Django ORM to SQLAlchemy session-based queries; FlightBlenderDatabaseReader/Writer now accept db: Session in constructor
• auth_helper/utils.py — requires_scopes decorator now uses fastapi.HTTPException instead of django.http.JsonResponse, extracts auth from request.headers instead of request.META
• common/data_definitions.py — removed gettext_lazy dependency, plain strings
• common/utils.py — removed DjangoJSONEncoder dependency
• Converted view modules: flight_feed_operations, rid_operations — Django views → FastAPI APIRouter endpoints with Depends(get_db) injection

What's NOT done

• View conversions for: scd_operations, uss_operations, geo_fence_operations, flight_declaration_operations, weather_monitoring_operations, surveillance_monitoring_operations, conformance_monitoring_operations
• FastAPI main app entry point (asgi.py with router mounting)
• Alembic migration setup
• Docker/entrypoint updates
• Test migration (Django TestCase → pytest + TestClient)
• Cleanup of Django artifacts (admin.py, apps.py, migrations/, manage.py)

Pattern

# Before (Django)
@api_view(["POST"])
@requires_scopes(["flightblender.write"])
def set_air_traffic(request, session_id):
    body = request.data
    my_database_writer = FlightBlenderDatabaseWriter()
    return JsonResponse(result, status=200)

# After (FastAPI)
@router.post("/set_air_traffic/{session_id}")
@requires_scopes(["flightblender.write"])
async def set_air_traffic(request: Request, session_id: str, db: Session = Depends(get_db)):
    body = json.loads(await request.body())
    my_database_writer = FlightBlenderDatabaseWriter(db)
    return JSONResponse(content=result, status_code=200)

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