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>

test/acceptance/scenario_coverage_test.go

@@ -38,6 +38,7 @@ var scenarioCoverage = map[string]string{
 
 	// connect_account.feature
 	"Connect a YouTube account":                              "TestCallbackExchangesAndRecordsConnection",
+	"Connecting an account discovers videos immediately":     "TestCallbackTriggersDiscovery",
 	"Tokens are never stored in the clear":                   "TestCallbackExchangesAndRecordsConnection",
 	"Revoking a connection stops watching but keeps history": "TestDisconnectRemovesTokenAndConnectionKeepsAccount",