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/models.py

@@ -48,6 +48,12 @@ class User(Base):
     # email -- see routers/device.py's frame_battery) go here; blank = no
     # email configured, both features silently no-op for this user.
     email: Mapped[str] = mapped_column(String, default="")
+    # Personal iCal/CalDAV .ics subscription URL (no OAuth) for calendar
+    # frame mode -- see calendar_feed.py. Setting this alone shows up
+    # nowhere: a linked frame only pulls this user's events in once
+    # they've also opted in on that frame's own Configuration -> Calendar
+    # card (UserFrame.calendar_included below).
+    calendar_ics_url: Mapped[str] = mapped_column(String, default="")
     created_at: Mapped[float] = mapped_column(Float, default=time.time)
 
     __table_args__ = (
@@ -139,6 +145,31 @@ class Frame(Base):
     # original always-on full-strength Floyd-Steinberg dithering.
     dither_strength: Mapped[float] = mapped_column(Float, default=1.0)
 
+    # -- calendar mode (see calendar_feed.py, calendar_render.py,
+    # routers/device.py's RENDERERS["calendar"]) --
+    calendar_view: Mapped[str] = mapped_column(String, default="agenda")  # "agenda" | "week" | "month"
+    # Agenda view only; reuses this frame's existing photos-mode album/
+    # queue, not a separate photo setup.
+    calendar_photo_inlay: Mapped[bool] = mapped_column(Boolean, default=False)
+    # How many periods (unit depends on calendar_view: days/weeks/months)
+    # NEXT/BACK have browsed from "today". Reset to 0 by the next normal
+    # (non-button) /frame/image request, and whenever calendar_view
+    # itself changes -- a stale offset means something different in a
+    # different view's units.
+    calendar_browse_offset: Mapped[int] = mapped_column(Integer, default=0)
+    # Throttled merge-fetch cache (see routers/common.py's
+    # get_or_refresh_calendar_events) -- same shape as the
+    # firmware_update_checked_at/firmware_gitea_latest_version pattern
+    # below. One shared cache for every included user's merged events,
+    # not per-user.
+    calendar_checked_at: Mapped[float] = mapped_column(Float, default=0.0)
+    calendar_cached_events: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
+    # "" when the last merge-fetch fully succeeded, else e.g. "1 of 2
+    # calendars unavailable" -- never names which user's feed failed, a
+    # shared household display shouldn't call out a specific person's
+    # outage to everyone who looks at it.
+    calendar_fetch_summary: Mapped[str] = mapped_column(String, default="")
+
     # -- state --
     current_asset_id: Mapped[str] = mapped_column(String, default="")
     current_asset_set_at: Mapped[float] = mapped_column(Float, default=0.0)
@@ -200,6 +231,14 @@ class UserFrame(Base):
         ForeignKey("frames.id", ondelete="CASCADE"), primary_key=True
     )
     created_at: Mapped[float] = mapped_column(Float, default=time.time)
+    # Explicit per-(user,frame) opt-in for calendar frame mode -- being
+    # linked to a frame does NOT by itself contribute this user's
+    # calendar to it (deliberate choice, not an oversight: each person's
+    # calendar is their own data to share or not, not something a
+    # frame's controller decides on their behalf). Meaningless if the
+    # user has no calendar_ics_url set. See routers/api_frames.py's
+    # api_calendar_included.
+    calendar_included: Mapped[bool] = mapped_column(Boolean, default=False)
 
 
 class PendingClaim(Base):