fix(backend): bound thumbnail generation and decode larger images

Thumbnails/previews are generated lazily per request with no concurrency
limit, and the imaging resize already fans out across every core — so
scrolling to a handful of large images spawned that many all-core,
hundreds-of-MB decodes at once and pegged the server. Add a generation
semaphore (THUMB_CONCURRENCY, default = half the CPUs) so only a bounded
number run at a time; queued requests wait and re-check the cache.

Also raise the decode cap from 64 Mpx to a configurable ~300 Mpx default
(THUMB_MAX_PIXELS) so genuinely large photos (e.g. 13000×17000 ≈ 221
Mpx) get a real thumbnail instead of falling back to a placeholder.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/integration/server_test.go

@@ -110,7 +110,7 @@ func setupSuite(t *testing.T) *harness {
 	thumbsDir := t.TempDir()
 	importDir := t.TempDir()
 
-	diskStorage, err := storage.NewDiskStorage(filesDir, thumbsDir, 160, 160, 1920, 1080)
+	diskStorage, err := storage.NewDiskStorage(filesDir, thumbsDir, 160, 160, 1920, 1080, 0, 0)
 	require.NoError(t, err)
 
 	// --- Repositories --------------------------------------------------------