feat(cmd): shared buildProcessor + wire immediate web summarization

Extract the engine wiring (YouTube source, AI-router summarizer, store sink)
into buildProcessor, shared by cmdRun and cmdServe. It returns (nil, nil) — not
an error — on incomplete config, which is the queue-only fallback for serve.
engineProcessor adapts the engine to web.Processor: load the video row, run the
engine, clear the manual queue flag on a produced summary (mirrors the runner).
cmdServe wires it onto web.App.Processor; cmdRun reuses buildProcessor so the
wiring is no longer duplicated.

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

cmd/tapir/processor_test.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"testing"
+
+	"gitea.d-ma.be/mathias/tapir/internal/config"
+)
+
+// TestBuildProcessorNilOnIncompleteConfig asserts the queue-only fallback: when a
+// required input is missing, buildProcessor returns (nil, nil) — never an error —
+// so the web UI degrades to queue-only instead of failing to start.
+func TestBuildProcessorNilOnIncompleteConfig(t *testing.T) {
+	// A complete config (the fields buildProcessor gates on). The store is nil:
+	// buildProcessor must not touch it on the incomplete paths, and the complete
+	// path only stores the pointer (no connection), so nil is fine for this test.
+	complete := config.Config{
+		GatewayURL:     "http://gw/v1",
+		YTClientID:     "id",
+		YTClientSecret: "secret",
+		SecretsFile:    "/tmp/secrets.json",
+	}
+
+	tests := []struct {
+		name    string
+		mutate  func(config.Config) config.Config
+		wantNil bool
+	}{
+		{"complete", func(c config.Config) config.Config { return c }, false},
+		{"no gateway url", func(c config.Config) config.Config { c.GatewayURL = ""; return c }, true},
+		{"no yt client id", func(c config.Config) config.Config { c.YTClientID = ""; return c }, true},
+		{"no yt client secret", func(c config.Config) config.Config { c.YTClientSecret = ""; return c }, true},
+		{"no secrets file", func(c config.Config) config.Config { c.SecretsFile = ""; return c }, true},
+		{"empty config", func(config.Config) config.Config { return config.Config{} }, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			engine, err := buildProcessor(tt.mutate(complete), nil)
+			if err != nil {
+				t.Fatalf("buildProcessor returned an error, want nil: %v", err)
+			}
+			if (engine == nil) != tt.wantNil {
+				t.Fatalf("engine == nil is %v, want %v", engine == nil, tt.wantNil)
+			}
+		})
+	}
+}