plugin(collab-buckets): collaborative buckets via relay

End-to-end shared buckets between relay tenants. Bucket metadata + members
sync through new relay routes (collab-buckets, collab-cards, collab-locks);
card content rides the existing inter-tenant encryption hop. Edit locks,
ownership transfer, LWW conflict stashing, undo for local-bucket destructive
actions, "Make collaborative…" upgrade path, and a resilient outbox for
relay-down create attempts. See plugins/collab-buckets/README.md and
TODO.md for the full picture.

Core hooks added so the plugin can integrate without monkey-patching:
- buckets.js context menu: collab-aware rename/color/delete + Members &
  owner / Make collaborative entries
- editor.js dispatches hyprcrd:editor-open / hyprcrd:editor-close for the
  per-card edit-lock heartbeat
- app.js exposes window.state for the plugin's MutationObserver
- index.php POST/PATCH/DELETE on /api/buckets refuse to mutate collab
  buckets directly (must go through the relay)
- Database.php adds buckets.collab_uid column + unique partial index
- import handler now restores color and collab_uid (was silently dropping
  color, which caused the test runner's auto-restore to wipe custom
  bucket colors)
- relay bootstrap creates collab_buckets, collab_bucket_members,
  collab_cards, collab_card_locks tables and prunes tombstones / expired
  locks on every request

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Database.php

@@ -139,5 +139,10 @@ class Database
         if (!in_array('color', $bcolNames)) {
             self::$pdo->exec("ALTER TABLE buckets ADD COLUMN color TEXT NOT NULL DEFAULT '#64748b'");
         }
+        // Collaborative buckets: plugin-managed relay UID. NULL for local buckets.
+        if (!in_array('collab_uid', $bcolNames)) {
+            self::$pdo->exec('ALTER TABLE buckets ADD COLUMN collab_uid TEXT DEFAULT NULL');
+            self::$pdo->exec('CREATE UNIQUE INDEX IF NOT EXISTS idx_buckets_collab_uid ON buckets(collab_uid) WHERE collab_uid IS NOT NULL');
+        }
     }
 }