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/code_shape.py

@@ -13,7 +13,7 @@ from sqlalchemy import (
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
-from scribe.models.base import TimestampMixin
+from scribe.models.base import TimestampMixin, iso
 
 # The classification vocabulary (note 2786). `unclassified` is the default and
 # THE todo state; every other status is a judgment, stamped with who made it.
@@ -153,18 +153,18 @@ class CodeShape(Base, TimestampMixin):
             "snippet_id": self.snippet_id,
             "reason": self.reason,
             "classified_by": self.classified_by,
-            "classified_at": self.classified_at.isoformat() if self.classified_at else None,
+            "classified_at": iso(self.classified_at),
             "first_seen_commit": self.first_seen_commit,
             "last_seen_commit": self.last_seen_commit,
-            "vanished_at": self.vanished_at.isoformat() if self.vanished_at else None,
+            "vanished_at": iso(self.vanished_at),
             "signature": self.signature,
             "body_sha": self.body_sha,
             "proposal": self.proposal,
             "classified_sha": self.classified_sha,
-            "recheck_at": self.recheck_at.isoformat() if self.recheck_at else None,
+            "recheck_at": iso(self.recheck_at),
             "diverges_from": self.diverges_from,
-            "created_at": self.created_at.isoformat(),
-            "updated_at": self.updated_at.isoformat(),
+            "created_at": iso(self.created_at),
+            "updated_at": iso(self.updated_at),
         }
 
 
@@ -221,5 +221,5 @@ class CodeShapeEvent(Base):
             "classified_by": self.classified_by,
             "reason": self.reason,
             "commit": self.commit,
-            "at": self.at.isoformat(),
+            "at": iso(self.at),
         }