extract routes from main.py into domain routers (Phase 4)

Split the 2,578-line main.py (90 routes) into 12 domain-specific router
modules under routes/. main.py is now a 45-line entry point.

New structure:
- app.py: FastAPI instance, CORS, startup/shutdown, safe_content_disposition
- routes/: health, profiles, channels, generations, history, transcription,
  stories, effects, audio, models, tasks, cuda
- services/cuda.py: moved from cuda_download.py

Also includes Phase 5 database/ package (from parallel agent):
- database/__init__.py re-exports all symbols for backward compat
- database/models.py, session.py, migrations.py, seed.py

All 90 routes verified registered and app imports cleanly.

backend/build_binary.py

@@ -67,7 +67,7 @@ def build_server(cuda=False):
         '--hidden-import', 'backend.utils.progress',
         '--hidden-import', 'backend.utils.hf_progress',
         '--hidden-import', 'backend.utils.validation',
-        '--hidden-import', 'backend.cuda_download',
+        '--hidden-import', 'backend.services.cuda',
         '--hidden-import', 'backend.effects',
         '--hidden-import', 'backend.utils.effects',
         '--hidden-import', 'backend.versions',