fix: resolve 4 bugs from QA round 6 (BUG-F/G/H/I)

BUG-F (P1): Fake webhook lookupInbox used JSONB @> operator on a TEXT
column, causing all fake webhooks to return "ignored". Cast
channel_config::jsonb before the @> operator.

BUG-G (P3): Vue Router history.state warning on Activity page. Four
call sites replaced history.state with null/{}, destroying Vue Router's
internal navigation state. Now all replaceState calls preserve
window.history.state.

BUG-H (P3): Inbox list showed stale data because cache_keys endpoint
returned hardcoded "0000000000" for inbox/label/team, so the frontend
IndexedDB cache never invalidated. Cache keys are now derived from
actual DB state (row count + MAX(updated_at)), with defensive fallback
for missing tables.

BUG-I (P3): All worker goroutines shared the same Redis consumer name,
so XINFO CONSUMERS showed 1 consumer instead of N. Each goroutine now
generates a unique consumer ID (workerID-index).

backend/internal/handler/webhook/fake_webhook.go

@@ -163,8 +163,9 @@ func (h *FakeWebhookHandler) lookupInbox(identifier string) (*model.Inbox, error
 	// PostgreSQL: use jsonb @> for a server-side, indexable query.
 	if h.db.Dialector.Name() == "postgres" {
 		var inbox model.Inbox
-		// channel_config @> '{"identifier":"<id>"}'
-		query := fmt.Sprintf(`channel_type = 'fake' AND channel_config @> '{"identifier":"%s"}'`, identifier)
+		// channel_config is TEXT, so cast to jsonb before using the @> operator.
+		// channel_config::jsonb @> '{"identifier":"<id>"}'
+		query := fmt.Sprintf(`channel_type = 'fake' AND channel_config::jsonb @> '{"identifier":"%s"}'`, identifier)
 		if err := h.db.Where(query).First(&inbox).Error; err != nil {
 			return nil, fmt.Errorf("fake inbox not found for identifier=%s: %w", identifier, err)
 		}