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/models.py

@@ -0,0 +1,155 @@
+"""ORM model definitions for the voicebox SQLite database."""
+
+from datetime import datetime
+import uuid
+
+from sqlalchemy import Column, String, Integer, Float, DateTime, Text, ForeignKey, Boolean
+from sqlalchemy.ext.declarative import declarative_base
+
+Base = declarative_base()
+
+
+class VoiceProfile(Base):
+    """Voice profile."""
+
+    __tablename__ = "profiles"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    name = Column(String, unique=True, nullable=False)
+    description = Column(Text)
+    language = Column(String, default="en")
+    avatar_path = Column(String, nullable=True)
+    effects_chain = Column(Text, nullable=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+
+class ProfileSample(Base):
+    """Audio sample attached to a voice profile."""
+
+    __tablename__ = "profile_samples"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    profile_id = Column(String, ForeignKey("profiles.id"), nullable=False)
+    audio_path = Column(String, nullable=False)
+    reference_text = Column(Text, nullable=False)
+
+
+class Generation(Base):
+    """A single TTS generation."""
+
+    __tablename__ = "generations"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    profile_id = Column(String, ForeignKey("profiles.id"), nullable=False)
+    text = Column(Text, nullable=False)
+    language = Column(String, default="en")
+    audio_path = Column(String, nullable=True)
+    duration = Column(Float, nullable=True)
+    seed = Column(Integer)
+    instruct = Column(Text)
+    engine = Column(String, default="qwen")
+    model_size = Column(String, nullable=True)
+    status = Column(String, default="completed")
+    error = Column(Text, nullable=True)
+    is_favorited = Column(Boolean, default=False)
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+
+class Story(Base):
+    """A story that sequences multiple generations."""
+
+    __tablename__ = "stories"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    name = Column(String, nullable=False)
+    description = Column(Text)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+
+class StoryItem(Base):
+    """Links a generation to a story at a specific timecode."""
+
+    __tablename__ = "story_items"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    story_id = Column(String, ForeignKey("stories.id"), nullable=False)
+    generation_id = Column(String, ForeignKey("generations.id"), nullable=False)
+    version_id = Column(String, ForeignKey("generation_versions.id"), nullable=True)
+    start_time_ms = Column(Integer, nullable=False, default=0)
+    track = Column(Integer, nullable=False, default=0)
+    trim_start_ms = Column(Integer, nullable=False, default=0)
+    trim_end_ms = Column(Integer, nullable=False, default=0)
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+
+class Project(Base):
+    """Audio studio project (JSON blob)."""
+
+    __tablename__ = "projects"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    name = Column(String, nullable=False)
+    data = Column(Text)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+
+class GenerationVersion(Base):
+    """A version of a generation's audio (original, processed, alternate takes)."""
+
+    __tablename__ = "generation_versions"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    generation_id = Column(String, ForeignKey("generations.id"), nullable=False)
+    label = Column(String, nullable=False)
+    audio_path = Column(String, nullable=False)
+    effects_chain = Column(Text, nullable=True)
+    source_version_id = Column(String, ForeignKey("generation_versions.id"), nullable=True)
+    is_default = Column(Boolean, default=False)
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+
+class EffectPreset(Base):
+    """Saved effect chain preset."""
+
+    __tablename__ = "effect_presets"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    name = Column(String, unique=True, nullable=False)
+    description = Column(Text, nullable=True)
+    effects_chain = Column(Text, nullable=False)
+    is_builtin = Column(Boolean, default=False)
+    sort_order = Column(Integer, default=100)
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+
+class AudioChannel(Base):
+    """Audio output channel (bus)."""
+
+    __tablename__ = "audio_channels"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    name = Column(String, nullable=False)
+    is_default = Column(Boolean, default=False)
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+
+class ChannelDeviceMapping(Base):
+    """Mapping between a channel and an OS audio device."""
+
+    __tablename__ = "channel_device_mappings"
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    channel_id = Column(String, ForeignKey("audio_channels.id"), nullable=False)
+    device_id = Column(String, nullable=False)
+
+
+class ProfileChannelMapping(Base):
+    """Many-to-many mapping between voice profiles and audio channels."""
+
+    __tablename__ = "profile_channel_mappings"
+
+    profile_id = Column(String, ForeignKey("profiles.id"), primary_key=True)
+    channel_id = Column(String, ForeignKey("audio_channels.id"), primary_key=True)