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/design_system.py

@@ -20,7 +20,7 @@ from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
-from scribe.models.base import SoftDeleteMixin, TimestampMixin
+from scribe.models.base import SoftDeleteMixin, TimestampMixin, iso
 
 
 class DesignSystem(Base, TimestampMixin, SoftDeleteMixin):
@@ -56,8 +56,8 @@ class DesignSystem(Base, TimestampMixin, SoftDeleteMixin):
             "description": self.description or "",
             "guidance": self.guidance or "",
             "parent_id": self.parent_id,
-            "created_at": self.created_at.isoformat() if self.created_at else None,
-            "updated_at": self.updated_at.isoformat() if self.updated_at else None,
+            "created_at": iso(self.created_at),
+            "updated_at": iso(self.updated_at),
         }
 
 
@@ -153,6 +153,6 @@ class DesignToken(Base, TimestampMixin, SoftDeleteMixin):
             "rationale": self.rationale,
             "supersedes": self.supersedes or [],
             "order_index": self.order_index,
-            "created_at": self.created_at.isoformat() if self.created_at else None,
-            "updated_at": self.updated_at.isoformat() if self.updated_at else None,
+            "created_at": iso(self.created_at),
+            "updated_at": iso(self.updated_at),
         }