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/models.py

@@ -307,13 +307,28 @@ 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")
     user = relationship("User")
 
 
+class CalendarColorPref(Base):
+    """A user's personal colour for a calendar they don't own — works for any
+    way a foreign calendar becomes visible (direct share, group calendar, or a
+    co-member's group-visible calendar). NULL/absent = the owner's colour."""
+
+    __tablename__ = "calendar_color_prefs"
+    __table_args__ = (
+        UniqueConstraint("calendar_id", "user_id", name="uq_calendar_color_pref"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    calendar_id = Column(Integer, ForeignKey("local_calendars.id"), nullable=False)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    color = Column(String(16), nullable=False)
+
+
 class Group(Base):
     __tablename__ = "groups"