feat(web): Processor port + in-flight ProcessingSet on App

Add the seam for immediate web-triggered summarization. Processor is the
optional single-video summarize port (nil = queue-only, unchanged behaviour);
ProcessingSet is an ephemeral, concurrency-safe set of in-flight (user,video)
ids so a status endpoint can show progress until the summary lands. State is
deliberately in-memory only — the DB holds the durable truth across restarts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/processing_test.go

@@ -0,0 +1,27 @@
+package web
+
+import "testing"
+
+func TestProcessingSetAddHasRemove(t *testing.T) {
+	var s ProcessingSet // zero value is usable
+
+	key := processingKey("user-1", "video-1")
+	if s.Has(key) {
+		t.Fatal("fresh set must not report a key as in-flight")
+	}
+
+	s.Add(key)
+	if !s.Has(key) {
+		t.Fatal("Add must mark the key in-flight")
+	}
+
+	// A different user with the same video id is a distinct key.
+	if s.Has(processingKey("user-2", "video-1")) {
+		t.Fatal("keys must be scoped by user")
+	}
+
+	s.Remove(key)
+	if s.Has(key) {
+		t.Fatal("Remove must clear the key")
+	}
+}