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/note_usage.py

@@ -1,15 +1,14 @@
-from datetime import datetime, timezone
-
-from sqlalchemy import DateTime, Index, Integer, Text
+from sqlalchemy import Index, Integer, Text
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
+from scribe.models.base import CreatedAtMixin, iso
 
 SURFACED = "surfaced"
 PULLED = "pulled"
 
 
-class NoteUsageEvent(Base):
+class NoteUsageEvent(Base, CreatedAtMixin):
     """One row per time a note was SURFACED to the agent, or PULLED in full.
 
     Answers the question RetrievalLog cannot: not "what did the ranker return
@@ -44,9 +43,6 @@ class NoteUsageEvent(Base):
     __tablename__ = "note_usage_events"
 
     id: Mapped[int] = mapped_column(primary_key=True)
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
     user_id: Mapped[int | None] = mapped_column(Integer, nullable=True)
     note_id: Mapped[int] = mapped_column(Integer, nullable=False)
     # 'surfaced' | 'pulled'
@@ -81,7 +77,7 @@ class NoteUsageEvent(Base):
     def to_dict(self) -> dict:
         return {
             "id": self.id,
-            "created_at": self.created_at.isoformat() if self.created_at else None,
+            "created_at": iso(self.created_at),
             "user_id": self.user_id,
             "note_id": self.note_id,
             "event": self.event,