refactor(background): parameterize sync queue and worker pool constructors

Split each constructor into a default-args wrapper and a config-accepting
variant so the sync queue interval/batch size and the worker pool size/
queue cap can be sourced from the settings registry at startup. These
values are constructed once at boot, so they are tagged requires_restart
in the admin UI.

queue.go:
- NewSyncQueueProcessorWithConfig(db, interval, batchSize) takes the
  flush interval and batch size as parameters; NewSyncQueueProcessor
  becomes a thin wrapper with the historical 5s / 50 defaults.

worker.go:
- NewWorkerWithConfig(numWorkers, queueCap, connManager) takes the
  queue capacity as a parameter; NewWorker becomes a thin wrapper with
  the historical cap of 100.

No behavior change for existing callers; main.go will switch to the
config-accepting variants in a follow-up wiring commit.

internal/sync/queue.go

@@ -74,11 +74,18 @@ type SyncQueueItem struct {
 }
 
 func NewSyncQueueProcessor(db *database.Queries) *SyncQueueProcessor {
+	return NewSyncQueueProcessorWithConfig(db, 5*time.Second, 50)
+}
+
+// NewSyncQueueProcessorWithConfig constructs a processor with the given flush
+// interval and batch size. Used at startup to source values from the settings
+// registry.
+func NewSyncQueueProcessorWithConfig(db *database.Queries, interval time.Duration, batchSize int) *SyncQueueProcessor {
 	return &SyncQueueProcessor{
 		db:           db,
 		progressChan: make(chan *ProgressUpdate, 100),
-		interval:     5 * time.Second,
-		batchSize:    50,
+		interval:     interval,
+		batchSize:    batchSize,
 	}
 }