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_version.py

@@ -2,7 +2,7 @@ from sqlalchemy import ARRAY, ForeignKey, Integer, Text
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
-from scribe.models.base import CreatedAtMixin
+from scribe.models.base import CreatedAtMixin, iso
 
 
 class NoteVersion(Base, CreatedAtMixin):
@@ -26,7 +26,7 @@ class NoteVersion(Base, CreatedAtMixin):
             "tags": self.tags or [],
             "pin_kind": self.pin_kind,
             "pin_label": self.pin_label,
-            "created_at": self.created_at.isoformat(),
+            "created_at": iso(self.created_at),
         }
         if include_body:
             d["body"] = self.body