S1: iso() datetime helper + one shared colour normalizer (colors.py)

M9 section S1, commit 3 — two more shared-toolkit pieces:

- common.iso(dt): the "x.isoformat() if x else None" idiom (repeated 20+ times
  across every serializer) as one helper. Adopted in Note.serialize() and the
  revision serializer; other serializers adopt it in their sections.
- colors.py: NOTE_COLORS (canonical, on the model) + a single normalize_color().
  notes.py now imports the palette + normalizer from here and drops its local
  copy. labels.py's identical LABEL_COLORS/_normalize_label_color fold into this
  in the Organize section (S3); sync in S4.

normalize_color and NOTE_COLORS remain importable from thoughtsync.notes (used by
tests + sync), so nothing downstream breaks. common has no in-app imports, so the
model→common→colors chain has no cycle. Behavior-preserving.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/models/note.py

@@ -8,6 +8,7 @@ from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
 from . import Base
+from ..common import iso
 
 # The Keep-style palette. Stored as a key string, so the actual tints live in the
 # frontend and can change without a schema migration.
@@ -82,8 +83,8 @@ class Note(Base):
             "pinned": self.pinned,
             "archived": self.archived,
             "trashed": self.deleted_at is not None,
-            "remind_at": self.remind_at.isoformat() if self.remind_at else None,
+            "remind_at": iso(self.remind_at),
             "recurrence": self.recurrence,
-            "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),
         }