add dev-only profiling with pprof, runtime/trace, and operation timing

Wire up Go's standard profiling toolkit so it's automatically available
in dev builds and completely absent from production. The profiling
package uses build tags (dev/!dev) to eliminate all pprof, trace, and
timing code from release binaries with zero new dependencies.

- backend/profiling: pprof HTTP server on :6060, /debug/trace endpoint,
  block/mutex profiling, and TimeOp helper for structured operation timing
- scripts/profile.sh: interactive menu-driven script that auto-selects
  free ports (8080-8089) so multiple profiles can be open simultaneously
- Instrumented key operations: app init, database init, player load/restore,
  queue set/restore
- Makefile targets: profile, profile-cpu, profile-heap, profile-trace
- .gitignore: exclude trace-*.out and *.pprof artifacts

backend/queue/queue.go

@@ -19,6 +19,7 @@ import (
 	"yellowjacket/backend/database"
 	"yellowjacket/backend/database/sql/sqlcgen"
 	"yellowjacket/backend/events"
+	"yellowjacket/backend/profiling"
 )
 
 // RepeatMode represents the queue repeat behavior.
@@ -611,6 +612,8 @@ func (q *Queue) handlePlayTracksNext(data ...any) {
 // resolved in the background. A generation counter ensures stale
 // background work is discarded if SetQueue is called again.
 func (q *Queue) SetQueue(filePaths []string, startIndex int) {
+	defer profiling.TimeOp(q.logger, "queue.SetQueue")()
+
 	gen := q.setQueueGen.Add(1)
 
 	if startIndex < 0 || startIndex >= len(filePaths) {
@@ -1592,6 +1595,8 @@ func (q *Queue) SaveState() {
 
 // RestoreState loads the queue state from the database.
 func (q *Queue) RestoreState() {
+	defer profiling.TimeOp(q.logger, "queue.RestoreState")()
+
 	q.mu.Lock()
 	defer q.mu.Unlock()