refactor(models): one iso() for every to_dict timestamp; mixins replace hand-rolled created_at/updated_at (#2827, milestone 296 area 3)

Reading all 28 models against each other: 54 `x.isoformat() if x else None` /
`x.isoformat()` expressions in 23 to_dict methods, in two guarded/unguarded
wordings, become iso() from models/base.py — uniform, and a row read before
flush serialises as null instead of raising. Rulebook / RulebookTopic / Rule
carried byte-identical copies of TimestampMixin's two columns;
InvitationToken / PasswordResetToken / NoteUsageEvent carried CreatedAtMixin's —
all six now use the mixin. AppLog and RetrievalLog keep their explicit
created_at, commented: their composite index orders on `created_at.desc()`,
which needs the column object in the class body. Schema-neutral (same column
definitions) — no migration.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/models/invitation.py

@@ -4,9 +4,10 @@ from sqlalchemy import Boolean, DateTime, ForeignKey, Index, Text
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
+from scribe.models.base import CreatedAtMixin
 
 
-class InvitationToken(Base):
+class InvitationToken(Base, CreatedAtMixin):
     __tablename__ = "invitation_tokens"
 
     id: Mapped[int] = mapped_column(primary_key=True)
@@ -15,9 +16,6 @@ class InvitationToken(Base):
     invited_by: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
     expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
     used: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
 
     __table_args__ = (
         Index("ix_invitation_tokens_token_hash", "token_hash"),