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

alembic/versions/0022_note_recurrence.py

@@ -0,0 +1,24 @@
+"""notes.recurrence (recurring reminders — M6 1908)
+
+Revision ID: 0022
+Revises: 0021
+Create Date: 2026-07-23
+
+Optional recurrence for a note's reminder (daily/weekly/monthly/yearly). On
+"complete", a recurring reminder advances remind_at to its next occurrence.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0022"
+down_revision = "0021"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("notes", sa.Column("recurrence", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("notes", "recurrence")