feat(events): recurring expansion, CalDAV pull sync, past search, reminders

- RRULE expansion: list_events now expands recurring events into
  individual occurrences within the query window using python-dateutil
- CalDAV pull sync: new caldav_sync.py + POST /api/events/sync route;
  imports remote events into the internal store by caldav_uid
- Past event search: search_events accepts include_past=true to search
  historical events; exposed in the LLM tool definition
- Internal reminders: migration 0037 adds reminder_minutes +
  reminder_sent_at columns; event_scheduler.py checks every 5 min and
  fires push notifications; CalDAV sync job runs hourly
- reminder_minutes now stored and returned in create/update routes + tools

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

alembic/versions/0037_add_event_reminders.py

@@ -0,0 +1,29 @@
+"""Add reminder_minutes and reminder_sent_at to events.
+
+Revision ID: 0037
+Revises: 0036
+Create Date: 2026-04-04
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0037"
+down_revision = "0036"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("events", sa.Column("reminder_minutes", sa.Integer(), nullable=True))
+    op.add_column(
+        "events",
+        sa.Column("reminder_sent_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("events", "reminder_sent_at")
+    op.drop_column("events", "reminder_minutes")