feat(serve): wire paste fetcher + connect-time onboarding burst

5c: app.Fetcher = a per-user YouTube videoFetcher, so POST /paste mounts and
resolves arbitrary-video metadata (Feature 2 goes live).

6: the connect trigger now runs an onboarding burst after discovery — summarize
up to TAPIR_ONBOARD_SUMMARIZE_COUNT of the user's newest unsummarized videos via
the gated Processor (Feature 1). Hard cap; explicit so it bypasses recency; every
fetch still through globalFetchGate. No-op when count=0 or queue-only.

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

cmd/tapir/discovery.go

@@ -32,7 +32,10 @@ func serialize(mu *sync.Mutex, run discoveryRunner) discoveryRunner {
 type discoveryTrigger struct {
 	ctx context.Context
 	run discoveryRunner
-	log *slog.Logger
+	// onboard, when set, runs after the discovery pass to summarize a capped number
+	// of the user's newest videos (Feature 1). Optional.
+	onboard func(ctx context.Context, userID string)
+	log     *slog.Logger
 }
 
 func (t *discoveryTrigger) Enqueue(userID string) {
@@ -40,5 +43,8 @@ func (t *discoveryTrigger) Enqueue(userID string) {
 		if _, err := t.run(t.ctx, userID); err != nil {
 			t.log.Warn("discovery: connect-triggered pass had errors", "user", userID, "err", err)
 		}
+		if t.onboard != nil {
+			t.onboard(t.ctx, userID)
+		}
 	}()
 }

cmd/tapir/main.go

@@ -210,7 +210,10 @@ func cmdServe(ctx context.Context, log *slog.Logger) error {
 			ClientSecret: cfg.YTClientSecret,
 			RedirectURL:  cfg.YTConnectRedirectURL,
 		}, secretStore, st, log)
-		log.Info("web youtube connect enabled", "redirect", cfg.YTConnectRedirectURL)
+		// Paste-a-URL (Feature 2): same YouTube credentials, per-user adapter built
+		// per request. Mounting the /paste route keys off app.Fetcher being set.
+		app.Fetcher = videoFetcher{cfg: cfg, secrets: secretStore}
+		log.Info("web youtube connect + paste enabled", "redirect", cfg.YTConnectRedirectURL)
 	}
 
 	// Immediate summarization for the web "Summarize" button. When the engine can
@@ -249,9 +252,33 @@ func cmdServe(ctx context.Context, log *slog.Logger) error {
 		// One lock shared by the scheduler and connect-triggered passes (#6) so
 		// they never fetch concurrently — the single-fetcher invariant (ADR-018).
 		runUser := serialize(&sync.Mutex{}, rawRunUser)
+		// Onboarding burst (Feature 1): after the connect-triggered discovery pass,
+		// summarize up to OnboardSummarizeCount of the user's NEWEST unsummarized
+		// videos so a fresh account gets real summaries in its first session. Hard
+		// cap; explicit, so it bypasses the recency window — but every fetch still
+		// goes through globalFetchGate via the Processor. No-op when disabled
+		// (count 0) or queue-only (no Processor).
+		onboard := func(ctx context.Context, userID string) {
+			if cfg.OnboardSummarizeCount <= 0 || app.Processor == nil {
+				return
+			}
+			ids, err := st.NewestUnsummarizedVideoIDs(ctx, userID, cfg.OnboardSummarizeCount)
+			if err != nil {
+				log.Warn("onboarding: list newest unsummarized", "user", userID, "err", err)
+				return
+			}
+			for _, id := range ids {
+				if err := app.Processor.ProcessVideo(ctx, userID, id); err != nil {
+					log.Warn("onboarding: summarize", "user", userID, "video", id, "err", err)
+				}
+			}
+			if len(ids) > 0 {
+				log.Info("onboarding burst complete", "user", userID, "summarized", len(ids), "cap", cfg.OnboardSummarizeCount)
+			}
+		}
 		if app.Connect != nil {
-			app.Connect.Discovery = &discoveryTrigger{ctx: ctx, run: runUser, log: log}
+			app.Connect.Discovery = &discoveryTrigger{ctx: ctx, run: runUser, onboard: onboard, log: log}
-			log.Info("connect-triggered discovery enabled")
+			log.Info("connect-triggered discovery enabled", "onboard_cap", cfg.OnboardSummarizeCount)
 		}
 		go runScheduler(ctx, cfg.DiscoveryInterval, st, runUser, log)
 	} else {

cmd/tapir/processor.go

@@ -11,9 +11,29 @@ import (
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/youtube"
 	"gitea.d-ma.be/mathias/tapir/internal/config"
 	"gitea.d-ma.be/mathias/tapir/internal/domain"
+	"gitea.d-ma.be/mathias/tapir/internal/ports"
 	"gitea.d-ma.be/mathias/tapir/internal/usecase"
+	"gitea.d-ma.be/mathias/tapir/internal/web"
 )
 
+// videoFetcher adapts the YouTube adapter to web.VideoFetcher for the paste flow
+// (Feature 2). It builds a per-user adapter bound to that user's token ref and
+// resolves a single video's metadata via the Data API — ungated; only the later
+// transcript fetch goes through globalFetchGate.
+type videoFetcher struct {
+	cfg     config.Config
+	secrets ports.SecretStore
+}
+
+func (f videoFetcher) FetchVideo(ctx context.Context, userID, videoID string) (domain.Video, error) {
+	a := youtube.New(youtube.Config{
+		ClientID:       f.cfg.YTClientID,
+		ClientSecret:   f.cfg.YTClientSecret,
+		TokenSecretRef: web.YouTubeTokenRef(userID),
+	}, f.secrets)
+	return a.VideoByID(ctx, userID, videoID)
+}
+
 // buildProcessor wires the summarization engine — YouTube source (captions-first),
 // AI-router summarizer, store sink — shared by `tapir run` and the web
 // "Summarize now" path so the wiring lives in one place. It returns (nil, nil) —