Add calendar frame mode data model (migration 7)

New columns, all ADD COLUMN with inert defaults -- no existing frame's
behavior changes until mode is explicitly switched to "calendar":

- users.calendar_ics_url: one personal iCal/CalDAV subscription per
  user, same shape as the existing per-user immich_url/immich_api_key.
- user_frames.calendar_included: explicit per-(user,frame) opt-in,
  default off. Being linked to a frame does not by itself contribute
  your calendar to it -- each person's calendar is their own data to
  share, not something a frame's controller decides on their behalf.
- frames.calendar_view/calendar_photo_inlay/calendar_browse_offset:
  per-frame display settings and NEXT/BACK navigation state.
- frames.calendar_checked_at/calendar_cached_events/calendar_fetch_summary:
  the throttled merge-fetch cache, same shape as the existing
  firmware_update_checked_at/firmware_gitea_latest_version pattern.

server/app/migration.py

@@ -80,6 +80,26 @@ def _migration_6(conn) -> None:
     conn.execute(text("ALTER TABLE frames ADD COLUMN dither_strength REAL NOT NULL DEFAULT 1.0"))
 
 
+def _migration_7(conn) -> None:
+    """Calendar frame mode: a personal ICS subscription per user
+    (users.calendar_ics_url), an explicit per-(user,frame) opt-in into a
+    frame's merged calendar (user_frames.calendar_included, default off
+    -- linking to a frame does not auto-include your calendar there),
+    and the frame-level view/inlay/browse-offset/cache settings calendar
+    mode needs (see calendar_feed.py, calendar_render.py,
+    routers/device.py's RENDERERS["calendar"]). Every new column has a
+    behavior-preserving default -- no existing frame's behavior changes
+    until its mode is actually switched to "calendar"."""
+    conn.execute(text("ALTER TABLE users ADD COLUMN calendar_ics_url TEXT NOT NULL DEFAULT ''"))
+    conn.execute(text("ALTER TABLE user_frames ADD COLUMN calendar_included INTEGER NOT NULL DEFAULT 0"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_view TEXT NOT NULL DEFAULT 'agenda'"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_photo_inlay INTEGER NOT NULL DEFAULT 0"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_browse_offset INTEGER NOT NULL DEFAULT 0"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_checked_at REAL NOT NULL DEFAULT 0.0"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_cached_events TEXT"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN calendar_fetch_summary TEXT NOT NULL DEFAULT ''"))
+
+
 MIGRATIONS = [
     (1, _migration_1),
     (2, _migration_2),
@@ -87,6 +107,7 @@ MIGRATIONS = [
     (4, _migration_4),
     (5, _migration_5),
     (6, _migration_6),
+    (7, _migration_7),
 ]