The export and the Markdown frontmatter write times through common.iso

common.iso is the server's one way to put a datetime on the wire, yet the JSON
export and the importer's Markdown frontmatter wrote out the
x.isoformat() if x else None idiom it replaces, seven times. Same output.

DRY pass #2, batch 4, F10 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/inkwell/notes/__init__.py

@@ -256,7 +256,7 @@ async def export_notes():
         payload: dict = {
             "app": "inkwell",
             "version": 1,
-            "exported_at": datetime.now(timezone.utc).isoformat(),
+            "exported_at": iso(datetime.now(timezone.utc)),
             "labels": [{"name": lb.name, "color": lb.color} for lb in all_labels],
             "notes": [],
         }
@@ -273,10 +273,10 @@ async def export_notes():
                         "body": n.body,
                         "pinned": n.pinned,
                         "archived": n.archived,
-                        "remind_at": n.remind_at.isoformat() if n.remind_at else None,
+                        "remind_at": iso(n.remind_at),
                         "recurrence": n.recurrence,
-                        "created_at": n.created_at.isoformat() if n.created_at else None,
-                        "updated_at": n.updated_at.isoformat() if n.updated_at else None,
+                        "created_at": iso(n.created_at),
+                        "updated_at": iso(n.updated_at),
                         "labels": [lb["name"] for lb in labels],
                         "attachments": [
                             {"file": f"attachments/{short}/{os.path.basename(a.path)}", "mime": a.mime} for a in atts

src/inkwell/notes/import_export.py

@@ -11,7 +11,7 @@ from datetime import datetime, timezone
 
 from sqlalchemy import select
 
-from ..common import parse_dt
+from ..common import iso, parse_dt
 from ..models.label import NoteLabel
 from ..models.note import Note
 from .helpers import (
@@ -37,9 +37,9 @@ def _note_markdown(note: Note, labels: list) -> str:
     if note.archived:
         fm.append("archived: true")
     if note.remind_at:
-        fm.append(f"remind_at: {note.remind_at.isoformat()}")
-    fm.append(f"created: {note.created_at.isoformat() if note.created_at else ''}")
-    fm.append(f"updated: {note.updated_at.isoformat() if note.updated_at else ''}")
+        fm.append(f"remind_at: {iso(note.remind_at)}")
+    fm.append(f"created: {iso(note.created_at) or ''}")
+    fm.append(f"updated: {iso(note.updated_at) or ''}")
     fm.append("---")
     fm.append("")
     # Body and checklist are no longer alternatives — a note can carry both, so both