docs(bdd): scenarios for paste-a-URL + onboarding burst, mapped to tests

Adds paste_url.feature (valid/invalid/not-found/dedup, +@pending no-captions)
and an onboarding-burst scenario on connect; all non-pending scenarios mapped in
the coverage gate to their existing Go tests.

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

test/acceptance/scenario_coverage_test.go

@@ -37,9 +37,16 @@ var scenarioCoverage = map[string]string{
 	"An authenticated user on the welcome page sees their way in and out": "TestWelcomeLoggedIn",
 
 	// connect_account.feature
-	"Connect a YouTube account":                              "TestCallbackExchangesAndRecordsConnection",
-	"Connecting an account discovers videos immediately":     "TestCallbackTriggersDiscovery",
-	"Tokens are never stored in the clear":                   "TestCallbackExchangesAndRecordsConnection",
+	"Connect a YouTube account":                          "TestCallbackExchangesAndRecordsConnection",
+	"Connecting an account discovers videos immediately": "TestCallbackTriggersDiscovery",
+	"Connecting summarizes my newest videos right away":  "TestNewestUnsummarizedVideoIDs",
+	"Tokens are never stored in the clear":               "TestCallbackExchangesAndRecordsConnection",
+
+	// paste_url.feature
+	"Paste a valid YouTube URL":                              "TestPasteValidURLAddsAndRequests",
+	"Pasting an invalid link is rejected":                    "TestPasteInvalidURLRejected",
+	"Pasting a video that cannot be found is honest":         "TestPasteVideoNotFound",
+	"Pasting the same video twice does not duplicate it":     "TestPasteDedupNoDuplicate",
 	"Revoking a connection stops watching but keeps history": "TestDisconnectRemovesTokenAndConnectionKeepsAccount",
 
 	// summarize_mode.feature