Replace deprecated datetime.utcnow() with datetime.now(UTC) throughout

Python 3.12 deprecates datetime.utcnow() with a DeprecationWarning and
it will be removed in a future release. Replace all call-site usages in
services, routes, and utils with datetime.now(UTC), and replace the
SQLAlchemy ORM column defaults (which used the bare function reference
datetime.utcnow) with lambda: datetime.now(UTC) so that the returned
objects are timezone-aware and consistent with Python best practice.

Affected: database/models.py, services/{stories,history,profiles,
channels,export_import}.py, routes/{profiles,tasks}.py, utils/tasks.py

backend/database/models.py

@@ -1,6 +1,6 @@
 """ORM model definitions for the voicebox SQLite database."""
 
-from datetime import datetime
+from datetime import UTC, datetime
 import uuid
 
 from sqlalchemy import Column, String, Integer, Float, DateTime, Text, ForeignKey, Boolean, JSON
@@ -44,8 +44,8 @@ class VoiceProfile(Base):
     # cloning metadata above).
     personality = Column(Text, nullable=True)
 
-    created_at = Column(DateTime, default=datetime.utcnow)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
+    updated_at = Column(DateTime, default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))
 
 
 class ProfileSample(Base):
@@ -82,7 +82,7 @@ class Generation(Base):
     # profile's personality LLM before TTS. Future sources (bulk import,
     # agent replies, etc.) can extend this.
     source = Column(String, nullable=False, default="manual")
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
 
 
 class Story(Base):
@@ -93,8 +93,8 @@ class Story(Base):
     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)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
+    updated_at = Column(DateTime, default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))
 
 
 class StoryItem(Base):
@@ -111,7 +111,7 @@ class StoryItem(Base):
     trim_start_ms = Column(Integer, nullable=False, default=0)
     trim_end_ms = Column(Integer, nullable=False, default=0)
     volume = Column(Float, nullable=False, default=1.0)
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
 
 
 class Project(Base):
@@ -122,8 +122,8 @@ class Project(Base):
     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)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
+    updated_at = Column(DateTime, default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))
 
 
 class GenerationVersion(Base):
@@ -138,7 +138,7 @@ class GenerationVersion(Base):
     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)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
 
 
 class EffectPreset(Base):
@@ -152,7 +152,7 @@ class EffectPreset(Base):
     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)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
 
 
 class AudioChannel(Base):
@@ -163,7 +163,7 @@ class AudioChannel(Base):
     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)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
 
 
 class ChannelDeviceMapping(Base):
@@ -218,7 +218,7 @@ class CaptureSettings(Base):
     chord_toggle_to_talk_keys = Column(
         JSON, nullable=False, default=default_toggle_to_talk_chord
     )
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    updated_at = Column(DateTime, default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))
 
 
 class GenerationSettings(Base):
@@ -231,7 +231,7 @@ class GenerationSettings(Base):
     crossfade_ms = Column(Integer, nullable=False, default=50)
     normalize_audio = Column(Boolean, nullable=False, default=True)
     autoplay_on_generate = Column(Boolean, nullable=False, default=True)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    updated_at = Column(DateTime, default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))
 
 
 class CloudSettings(Base):
@@ -276,8 +276,8 @@ class MCPClientBinding(Base):
     # (rewrite) before TTS by default. Callers can still override per call.
     default_personality = Column(Boolean, nullable=False, default=False)
     last_seen_at = Column(DateTime, nullable=True)
-    created_at = Column(DateTime, default=datetime.utcnow)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
+    updated_at = Column(DateTime, default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))
 
 
 class Capture(Base):
@@ -300,4 +300,4 @@ class Capture(Base):
     stt_model = Column(String, nullable=True)
     llm_model = Column(String, nullable=True)
     refinement_flags = Column(Text, nullable=True)  # JSON blob
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))