refactor(channelserver): replace global stagesLock with sync.Map-backed StageMap

The global stagesLock sync.RWMutex protected map[string]*Stage, causing
all stage operations to contend on a single lock even for unrelated
stages. Any stage creation or deletion blocked all reads server-wide.

Replace with a typed StageMap wrapper around sync.Map which provides
lock-free reads and allows concurrent writes to disjoint keys. Per-stage
sync.RWMutex remains unchanged for protecting individual stage state.

StageMap exposes Get, GetOrCreate, StoreIfAbsent, Store, Delete, and
Range methods. Updated ~50 call sites across 6 production files and
9 test files.

server/channelserver/session_lifecycle_integration_test.go

@@ -582,7 +582,6 @@ func createTestServerWithDB(t *testing.T, db *sqlx.DB) *Server {
 	server := &Server{
 		db:              db,
 		sessions:        make(map[net.Conn]*Session),
-		stages:          make(map[string]*Stage),
 		userBinary: NewUserBinaryStore(),
 		minidata:   NewMinidataStore(),
 		semaphore:       make(map[string]*Semaphore),