fix(sharing): per-user calendar colour works for every sharing path

The recipient colour was stored on calendar_shares, so it only worked for
DIRECT shares. A calendar made visible through a group (a co-member's
group_visible_calendar_id) has no CalendarShare row, so the colour endpoint
returned 403 and nothing was saved — the reported "colour picker opens but the
colour stays the same" for a group-shared calendar.

Replace the share-scoped colour with a general per-user override table
(calendar_color_prefs, keyed by user+calendar). PUT /calendars/{id}/color now
accepts any calendar the user can read (readable_local_calendar_ids covers
direct shares, group calendars AND group-visible), and the merge read + calendar
list apply the override for all of them. Owners still set the shared colour.
The new table is created by create_all; the old calendar_shares.color is unused.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/permissions.py

@@ -163,3 +163,14 @@ def readable_local_calendar_ids(db: Session, user: models.User) -> list[int]:
     ids.update(c.id for c in co_member_group_visible_calendars(db, user))
 
     return list(ids)
+
+
+def color_prefs_for(db: Session, user_id: int) -> dict[int, str]:
+    """Map calendar_id -> the user's personal colour for calendars they don't
+    own (any sharing path). Empty when the user set no overrides."""
+    return {
+        p.calendar_id: p.color
+        for p in db.query(models.CalendarColorPref).filter(
+            models.CalendarColorPref.user_id == user_id
+        )
+    }