M6 1908a: recurring reminders + complete/snooze (backend, no web-push)

Per operator: skip Web Push; build the rest of reminder delivery. This is
the client-agnostic half — the model + logic that foreground/native
delivery drives.

- notes.recurrence (migration 0022): daily/weekly/monthly/yearly or null.
  update_note accepts it (cleared when the reminder is cleared); rides
  export/import + sync push. Serialized on the note.
- Pure next_occurrence(remind_at, recurrence, after): the next fire strictly
  after `after`, rolling past missed occurrences; _add_months clamps the day
  to the target month (Jan 31 → Feb 28).
- POST /api/notes/<id>/reminder/complete — a recurring reminder advances to
  its next occurrence; a one-off clears. POST .../reminder/snooze {minutes}
  → remind_at = now + minutes (1 min .. 30 days).

No VAPID / push-subscription / service-worker — foreground + native delivery
land in the UI commit and the native clients.

Tests (DB-free): normalize_recurrence; next_occurrence (daily/weekly/
monthly-clamp/skip-missed/yearly/none); complete + snooze auth-guards.

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

@@ -53,8 +53,11 @@ class Note(Base):
     archived: Mapped[bool] = mapped_column(Boolean(), nullable=False, server_default=func.false())
     # Soft delete: non-null => in Trash. Restore sets it back to null.
     deleted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
-    # Optional reminder time (surfaced in the Reminders view; no push in M3).
+    # Optional reminder time (surfaced in the Reminders view + foreground delivery).
     remind_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    # Optional recurrence for the reminder: daily | weekly | monthly | yearly (else null).
+    # On "complete", a recurring reminder advances remind_at to its next occurrence.
+    recurrence: Mapped[str | None] = mapped_column(Text(), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
@@ -80,6 +83,7 @@ class Note(Base):
             "archived": self.archived,
             "trashed": self.deleted_at is not None,
             "remind_at": self.remind_at.isoformat() if self.remind_at else None,
+            "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,
         }