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/rulebook.py

@@ -4,10 +4,10 @@ from sqlalchemy import BigInteger, Boolean, Column, DateTime, ForeignKey, Index,
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
-from scribe.models.base import SoftDeleteMixin
+from scribe.models.base import SoftDeleteMixin, TimestampMixin, iso
 
 
-class Rulebook(Base, SoftDeleteMixin):
+class Rulebook(Base, TimestampMixin, SoftDeleteMixin):
     __tablename__ = "rulebooks"
 
     id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
@@ -19,14 +19,6 @@ class Rulebook(Base, SoftDeleteMixin):
     always_on: Mapped[bool] = mapped_column(
         Boolean, default=False, nullable=False, server_default="false"
     )
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
-    updated_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True),
-        default=lambda: datetime.now(timezone.utc),
-        onupdate=lambda: datetime.now(timezone.utc),
-    )
 
     def to_dict(self) -> dict:
         return {
@@ -35,12 +27,12 @@ class Rulebook(Base, SoftDeleteMixin):
             "title": self.title,
             "description": self.description or "",
             "always_on": self.always_on,
-            "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),
         }
 
 
-class RulebookTopic(Base, SoftDeleteMixin):
+class RulebookTopic(Base, TimestampMixin, SoftDeleteMixin):
     __tablename__ = "rulebook_topics"
     # Partial unique: a title is unique among LIVE topics in a rulebook, so a
     # trashed topic doesn't block recreating/restoring the same title.
@@ -58,14 +50,6 @@ class RulebookTopic(Base, SoftDeleteMixin):
     title: Mapped[str] = mapped_column(Text)
     description: Mapped[str | None] = mapped_column(Text, nullable=True)
     order_index: Mapped[int] = mapped_column(Integer, default=0)
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
-    updated_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True),
-        default=lambda: datetime.now(timezone.utc),
-        onupdate=lambda: datetime.now(timezone.utc),
-    )
 
     def to_dict(self) -> dict:
         return {
@@ -74,12 +58,12 @@ class RulebookTopic(Base, SoftDeleteMixin):
             "title": self.title,
             "description": self.description 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),
         }
 
 
-class Rule(Base, SoftDeleteMixin):
+class Rule(Base, TimestampMixin, SoftDeleteMixin):
     __tablename__ = "rules"
     # Partial unique: title unique among LIVE rules in a topic (soft-deleted
     # rules don't block recreating/restoring the same title).
@@ -109,14 +93,6 @@ class Rule(Base, SoftDeleteMixin):
     why: Mapped[str | None] = mapped_column(Text, nullable=True)
     how_to_apply: Mapped[str | None] = mapped_column(Text, nullable=True)
     order_index: Mapped[int] = mapped_column(Integer, default=0)
-    created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
-    )
-    updated_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True),
-        default=lambda: datetime.now(timezone.utc),
-        onupdate=lambda: datetime.now(timezone.utc),
-    )
 
     def to_dict(self) -> dict:
         return {
@@ -128,8 +104,8 @@ class Rule(Base, SoftDeleteMixin):
             "why": self.why or "",
             "how_to_apply": self.how_to_apply 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),
         }