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/app_log.py

@@ -4,6 +4,7 @@ from sqlalchemy import DateTime, Float, Index, Integer, Text
 from sqlalchemy.orm import Mapped, mapped_column
 
 from scribe.models import Base
+from scribe.models.base import iso
 
 
 class AppLog(Base):
@@ -20,6 +21,9 @@ class AppLog(Base):
     duration_ms: Mapped[float | None] = mapped_column(Float, nullable=True)
     ip_address: Mapped[str | None] = mapped_column(Text, nullable=True)
     details: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # Declared here rather than via CreatedAtMixin on purpose: the composite
+    # index below orders on `created_at.desc()`, which needs the column object
+    # in this class body — a mixin's column is not in scope there.
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
@@ -44,5 +48,5 @@ class AppLog(Base):
             "duration_ms": self.duration_ms,
             "ip_address": self.ip_address,
             "details": self.details,
-            "created_at": self.created_at.isoformat() if self.created_at else None,
+            "created_at": iso(self.created_at),
         }