Fix #11: Replace deprecated datetime.utcnow with timezone-aware UTC

- Import timezone from datetime
- Add _utcnow() helper using datetime.now(timezone.utc)
- Replace all default=datetime.utcnow column defaults
- Fixes deprecation warnings on Python 3.12+

models.py

@@ -7,10 +7,14 @@ users    – Registered accounts
 messages – Encrypted PM history (user↔user and user↔AI)
 """
 
-from datetime import datetime
+from datetime import datetime, timezone
 from database import db
 
 
+def _utcnow():
+    return datetime.now(timezone.utc)
+
+
 class User(db.Model):
     __tablename__ = "users"
 
@@ -21,7 +25,7 @@ class User(db.Model):
     has_ai_access    = db.Column(db.Boolean, default=False, nullable=False)
     ai_messages_used = db.Column(db.Integer, default=0, nullable=False)
     is_verified      = db.Column(db.Boolean, default=False, nullable=False)
-    created_at       = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
+    created_at       = db.Column(db.DateTime, default=_utcnow, nullable=False)
 
     sent_messages     = db.relationship(
         "Message", foreign_keys="Message.sender_id",
@@ -52,7 +56,7 @@ class UserIgnore(db.Model):
     id         = db.Column(db.Integer, primary_key=True)
     ignorer_id = db.Column(db.Integer, db.ForeignKey("users.id"), nullable=False)
     ignored_id = db.Column(db.Integer, db.ForeignKey("users.id"), nullable=False)
-    created_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
+    created_at = db.Column(db.DateTime, default=_utcnow, nullable=False)
 
     __table_args__ = (
         db.Index("ix_ignore_pair", "ignorer_id", "ignored_id", unique=True),
@@ -70,7 +74,7 @@ class Message(db.Model):
     encrypted_content = db.Column(db.Text, nullable=False)
     # AES-GCM nonce / IV – base64 encoded (12 bytes → 16 chars)
     nonce             = db.Column(db.String(64), nullable=False)
-    timestamp         = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
+    timestamp         = db.Column(db.DateTime, default=_utcnow, nullable=False)
 
     __table_args__ = (
         # Composite indices for the two common query patterns
@@ -90,7 +94,7 @@ class Ban(db.Model):
     username   = db.Column(db.String(20), nullable=False, index=True)
     ip         = db.Column(db.String(45), nullable=True, index=True)
     reason     = db.Column(db.String(255), nullable=True)
-    created_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
+    created_at = db.Column(db.DateTime, default=_utcnow, nullable=False)
 
 
 class Mute(db.Model):
@@ -99,4 +103,4 @@ class Mute(db.Model):
 
     id         = db.Column(db.Integer, primary_key=True)
     username   = db.Column(db.String(20), unique=True, nullable=False, index=True)
-    created_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
+    created_at = db.Column(db.DateTime, default=_utcnow, nullable=False)