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/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")