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.py

@@ -6,7 +6,7 @@ from sqlalchemy.dialects.postgresql import ARRAY, JSONB
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
-from scribe.models.base import TimestampMixin, SoftDeleteMixin
+from scribe.models.base import SoftDeleteMixin, TimestampMixin, iso
 
 
 class TaskStatus(str, enum.Enum):
@@ -105,18 +105,14 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
             "milestone_id": self.milestone_id,
             "status": self.status,
             "priority": self.priority,
-            "due_date": self.due_date.isoformat() if self.due_date else None,
-            "started_at": self.started_at.isoformat() if self.started_at else None,
-            "completed_at": self.completed_at.isoformat() if self.completed_at else None,
+            "due_date": iso(self.due_date),
+            "started_at": iso(self.started_at),
+            "completed_at": iso(self.completed_at),
             "recurrence_rule": self.recurrence_rule,
-            "recurrence_next_spawn_at": (
-                self.recurrence_next_spawn_at.isoformat()
-                if self.recurrence_next_spawn_at
-                else None
-            ),
+            "recurrence_next_spawn_at": iso(self.recurrence_next_spawn_at),
             "is_task": self.is_task,
             "note_type": self.note_type or "note",
             "task_kind": self.task_kind,
-            "created_at": self.created_at.isoformat(),
-            "updated_at": self.updated_at.isoformat(),
+            "created_at": iso(self.created_at),
+            "updated_at": iso(self.updated_at),
         }