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: