feat(discovery): trigger a discovery pass on YouTube connect

A newly connected account showed no videos until the next 2h scheduled pass —
the gap that made onboarding look broken (a second user connected, saw nothing,
read as failure). The connect callback now fires an out-of-band discovery pass
for the connecting user, so videos appear promptly.

Concurrency: scheduled and connect-triggered passes share one lock (serialize),
preserving the single-fetcher invariant (ADR-018). A trigger interleaves between
the scheduler's per-user passes rather than fetching concurrently or waiting for
a whole pass. The trigger runs on the server ctx (survives the redirect) and is
non-blocking for the request goroutine.

Scope: connect-trigger only. The optional login-refresh / "Discover now" button
from #6 are intentionally not built — an unconditional login hook risks 429
storms (per the ticket's own recommendation); defer until wanted.

TDD: TestCallbackTriggersDiscovery, TestSerializeRunsOneAtATime,
TestDiscoveryTriggerEnqueueRunsUser; new BDD scenario mapped.

Refs #6

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

internal/web/connect.go

@@ -23,6 +23,15 @@ type Connections interface {
 	UpsertConnection(ctx context.Context, userID string, c store.Connection) error
 }
 
+// DiscoveryTrigger requests an out-of-band discovery pass for a user. The connect
+// flow fires it the moment a YouTube account is linked so videos appear promptly
+// instead of waiting for the next scheduled pass (#6). Enqueue must be
+// non-blocking and safe to call from the request goroutine; the implementation
+// owns serialization with the scheduler (one pass at a time). nil = no trigger.
+type DiscoveryTrigger interface {
+	Enqueue(userID string)
+}
+
 // connectStateTTL bounds how long a generated CSRF state is valid between the
 // connect redirect and the provider callback.
 const connectStateTTL = 10 * time.Minute
@@ -43,6 +52,10 @@ type ConnectHandler struct {
 	Conns   Connections
 	Log     *slog.Logger
 
+	// Discovery, when set, is fired after a successful connect so the new
+	// connection's videos are discovered immediately (#6). Optional.
+	Discovery DiscoveryTrigger
+
 	states *connectStateStore
 	now    func() time.Time
 }
@@ -134,6 +147,12 @@ func (h *ConnectHandler) handleCallback(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
+	// Discover this user's videos now rather than waiting for the next scheduled
+	// pass (#6). Non-blocking; the trigger serializes with the scheduler.
+	if h.Discovery != nil {
+		h.Discovery.Enqueue(userID)
+	}
+
 	setFlash(w, flashConnected)
 	http.Redirect(w, r, "/", http.StatusSeeOther)
 }