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/database/__init__.py

@@ -0,0 +1,44 @@
+"""Database package — ORM models, session management, and migrations.
+
+Re-exports all public symbols so that ``from .database import get_db``
+and ``from .database import Generation as DBGeneration`` continue to work
+without changing any importers.
+"""
+
+from .models import (
+    Base,
+    AudioChannel,
+    ChannelDeviceMapping,
+    EffectPreset,
+    Generation,
+    GenerationVersion,
+    ProfileChannelMapping,
+    ProfileSample,
+    Project,
+    Story,
+    StoryItem,
+    VoiceProfile,
+)
+from .session import engine, SessionLocal, _db_path, init_db, get_db
+
+__all__ = [
+    # Models
+    "Base",
+    "AudioChannel",
+    "ChannelDeviceMapping",
+    "EffectPreset",
+    "Generation",
+    "GenerationVersion",
+    "ProfileChannelMapping",
+    "ProfileSample",
+    "Project",
+    "Story",
+    "StoryItem",
+    "VoiceProfile",
+    # Session
+    "engine",
+    "SessionLocal",
+    "_db_path",
+    "init_db",
+    "get_db",
+]