docs: update developer snippets to datetime.now(UTC)

docs/content/docs/developer/history.mdx

@@ -28,7 +28,7 @@ class Generation(Base):
     status = Column(String, default="completed")  # pending | completed | failed
     error = Column(Text, nullable=True)
     is_favorited = Column(Boolean, default=False)
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
 ```
 
 Each generation can also have multiple **generation versions** — processed variants with different effects chains applied. The original (`clean`) version plus any number of processed versions live in a separate `generation_versions` table. See [Effects Pipeline](/developer/effects-pipeline).
@@ -71,7 +71,7 @@ async def create_generation(
         duration=duration,
         seed=seed,
         instruct=instruct,
-        created_at=datetime.utcnow(),
+        created_at=datetime.now(UTC),
     )
     
     db.add(db_generation)

docs/content/docs/developer/voice-profiles.mdx

@@ -44,8 +44,8 @@ class VoiceProfile(Base):
     design_prompt = Column(Text, nullable=True)      # text description — only for designed (reserved)
     default_engine = Column(String, nullable=True)   # auto-selected engine, locked for preset
 
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime, default=lambda: datetime.now(UTC))
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    updated_at = Column(DateTime, default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC))
 ```
 
 The `voice_type` column discriminates the three flavors: