feat(sharing): label shared personal calendars by owner + drop group title prefix

Two group/sharing display fixes, both server-side so every client benefits:

1. A personal calendar shared WITH a user showed only its raw name
   ("Persönlich"), indistinguishable from the user's own. The merge read now
   relabels a shared *personal* calendar under the owner's display name (so
   Guido's "Persönlich" reads as "Guido" for recipients) and adds read_only:true
   when the share isn't read_write. Group calendars are excluded — they keep
   their own name and stay writable for members.

2. The combined group view prefixed every foreign event with the owner's first
   name ("Guido: …"). Each member already has a distinct display_color, so the
   prefix was redundant. _decorate_title now returns the raw title; display_title
   stays non-empty so clients' legacy prefix fallback never triggers. The change
   takes effect on already-installed clients with no app update.

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

backend/routers/caldav_router.py

@@ -352,6 +352,21 @@ def get_events(
         .all()
     ) if readable_ids else []
     name_cache = {u.id: (u.display_name or u.username) for u in db.query(models.User).all()}
+    # A personal calendar shared WITH the current user is relabelled under the
+    # owner's name (so Guido's "Persönlich" reads as "Guido" for his mum) and
+    # flagged read_only unless the share grants write. Group calendars are
+    # excluded — they keep their own name and stay writable for members.
+    shares_map = {
+        s.calendar_id: s.permission
+        for s in db.query(models.CalendarShare).filter(
+            models.CalendarShare.user_id == current_user.id
+        )
+    }
+    group_cal_ids = {
+        r[0] for r in db.query(models.GroupCalendar.calendar_id).filter(
+            models.GroupCalendar.calendar_id.in_(readable_ids)
+        )
+    } if readable_ids else set()
     # Cache each owner's private-event visibility (one lookup per owner, not per event).
     vis_cache: dict = {}
 
@@ -361,6 +376,13 @@ def get_events(
         return vis_cache[uid]
 
     for local_cal in local_calendars:
+        # Decoration for a personal calendar shared with (not owned by) me.
+        is_shared_personal = (
+            local_cal.user_id != current_user.id
+            and local_cal.id not in group_cal_ids
+        )
+        shared_owner_name = name_cache.get(local_cal.user_id) if is_shared_personal else None
+        shared_read_only = is_shared_personal and shares_map.get(local_cal.id) != "read_write"
         local_events = (
             db.query(models.LocalEvent)
             .filter(
@@ -391,6 +413,10 @@ def get_events(
             else:
                 built = [build_local_event_dict(ev, local_cal, rrule=None, creator=creator)]
             for b in built:
+                if shared_owner_name:
+                    b["calendar_name"] = shared_owner_name
+                if shared_read_only:
+                    b["read_only"] = True
                 b = apply_event_privacy(
                     b, owner_id=owner_id, is_private=is_priv,
                     requester_id=current_user.id, visibility=visibility,

backend/routers/groups_router.py

@@ -317,17 +317,12 @@ def _first_name(name: Optional[str]) -> str:
 
 def _decorate_title(title: str, *, is_group: bool, creator: Optional[dict],
                     owner: Optional[dict], me_id: int) -> str:
-    """Server-side display title for the combined view so every client (web,
-    iOS, Android) renders identically: another member's / creator's first name
-    is prefixed. No icon glyph is embedded — group icons are semantic keys the
-    clients render as native vector icons, and group-calendar events are
-    distinguished by their (group) colour. The raw `title` stays for editing."""
-    if is_group:
-        if creator and creator.get("id") is not None and creator.get("id") != me_id:
-            return f"{_first_name(creator.get('display_name'))}: {title}"
-        return title
-    if owner and owner.get("id") is not None and owner.get("id") != me_id:
-        return f"{_first_name(owner.get('display_name'))}: {title}"
+    """Server-side display title for the combined view. The former owner/creator
+    first-name prefix ("Guido: …") was dropped: each member already has a
+    distinct colour, so the prefix was redundant noise. We still return a
+    non-empty `display_title` (== raw title) so the clients' legacy fallback —
+    which rebuilds a prefix when `display_title` is empty — never kicks in.
+    `display_color` continues to carry the per-person colour."""
     return title