feat(sharing): recipients can recolour a shared calendar (per-user, no rename)

A share recipient couldn't change anything on a shared calendar (update_calendar
is owner-only → 404), yet the clients still showed a colour picker for it.

Add a per-recipient colour: new nullable calendar_shares.color column (+ migration).
New PUT /calendars/{id}/color endpoint sets the calendar colour for the owner
(global) or, for a recipient, only their own share colour — never the name, so
recipients can recolour but not rename. The merge read and the calendar list now
prefer the recipient's share colour over the owner's (NULL = owner's colour).

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

backend/main.py

@@ -243,6 +243,13 @@ def _migrate():
             logging.info("Migration: added etag to local_events")
         except Exception:
             pass
+        # Per-recipient colour override for a shared calendar (NULL = owner's colour).
+        try:
+            conn.execute(text("ALTER TABLE calendar_shares ADD COLUMN color VARCHAR(16)"))
+            conn.commit()
+            logging.info("Migration: added color to calendar_shares")
+        except Exception:
+            pass
 
         # Hide a user from sharing/group pickers (admin management still shows them).
         try:

backend/models.py

@@ -307,6 +307,7 @@ class CalendarShare(Base):
     calendar_id = Column(Integer, ForeignKey("local_calendars.id"), nullable=False)
     user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
     permission = Column(String(20), default="read")  # 'read' | 'read_write'
+    color = Column(String(16), nullable=True)  # recipient's own colour; NULL = owner's
     created_at = Column(String(50), nullable=True)  # ISO 8601
 
     calendar = relationship("LocalCalendar")

backend/routers/caldav_router.py

@@ -356,8 +356,8 @@ def get_events(
     # 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
+    shares_by_cal = {
+        s.calendar_id: s
         for s in db.query(models.CalendarShare).filter(
             models.CalendarShare.user_id == current_user.id
         )
@@ -381,8 +381,10 @@ def get_events(
             local_cal.user_id != current_user.id
             and local_cal.id not in group_cal_ids
         )
+        share = shares_by_cal.get(local_cal.id) if is_shared_personal else None
         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"
+        shared_read_only = is_shared_personal and (share.permission if share else None) != "read_write"
+        shared_color = share.color if share else None
         local_events = (
             db.query(models.LocalEvent)
             .filter(
@@ -417,6 +419,8 @@ def get_events(
                     b["calendar_name"] = shared_owner_name
                 if shared_read_only:
                     b["read_only"] = True
+                if shared_color:
+                    b["calendarColor"] = shared_color
                 b = apply_event_privacy(
                     b, owner_id=owner_id, is_private=is_priv,
                     requester_id=current_user.id, visibility=visibility,

backend/routers/local_router.py

@@ -70,11 +70,13 @@ class ShareCreate(BaseModel):
 
 def _cal_dict(cal: models.LocalCalendar, *, owned: bool = True,
               shared_by: Optional[str] = None, permission: Optional[str] = None,
+              color_override: Optional[str] = None,
               request: Optional[Request] = None) -> dict:
     d = {
         "id": cal.id,
         "name": cal.name,
-        "color": cal.color,
+        # A recipient's own colour for a shared calendar wins over the owner's.
+        "color": color_override or cal.color,
         "enabled": cal.enabled,
         "reminders_enabled": bool(cal.reminders_enabled),
         "caldav_published": bool(cal.caldav_published),
@@ -150,6 +152,7 @@ def list_calendars(
             cal, owned=False,
             shared_by=(owner.display_name or owner.username) if owner else None,
             permission=share.permission,
+            color_override=share.color,
         )
         if cal.id in group_cal_map:
             d["group"] = True
@@ -246,6 +249,33 @@ def update_calendar(
     return _cal_dict(cal, owned=True, request=request)
 
 
+class CalendarColorUpdate(BaseModel):
+    color: str
+
+
+@router.put("/calendars/{calendar_id}/color")
+def set_calendar_color(
+    calendar_id: int,
+    data: CalendarColorUpdate,
+    db: Session = Depends(get_db),
+    current_user: models.User = Depends(get_current_user),
+):
+    """Set a calendar's colour. The owner changes the calendar's colour for
+    everyone; a share recipient sets only their OWN per-user colour (stored on
+    the share) without touching the owner's. Recipients may recolour but never
+    rename a shared calendar."""
+    cal = permissions.accessible_local_calendar(db, current_user, calendar_id)
+    if cal.user_id == current_user.id:
+        cal.color = data.color
+    else:
+        share = permissions._share_for(db, calendar_id, current_user.id)
+        if share is None:
+            raise HTTPException(403, "Only the owner or a share recipient can set the colour")
+        share.color = data.color
+    db.commit()
+    return {"ok": True, "color": data.color}
+
+
 @router.post("/calendars/{calendar_id}/dav-token/rotate")
 def rotate_dav_token(
     calendar_id: int,