feat(domain): add SourceRateLimited transcript source

429 from the caption endpoint means the IP is rate-limited (retry later),
not that the video has no captions. Distinguishing it from SourceNone is the
prerequisite for the runner's backoff/retry logic.

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

internal/web/view.go

@@ -535,6 +535,11 @@ a.btn, a.btn:visited { color: var(--accent-fg); }
 .empty { text-align: center; color: var(--muted); padding: var(--s5) var(--s4); border: 1px dashed var(--line); border-radius: var(--radius); background: var(--card); }
 .empty strong { display: block; color: var(--fg); font-size: 1.05rem; margin-bottom: var(--s2); }
 .empty code { background: var(--accent-weak); color: var(--accent); padding: .1rem .35rem; border-radius: .3rem; }
+.empty p { margin: var(--s3) 0 0; }
+/* connected-but-empty: a distinct accent callout, not a muted blank state, so a
+   fresh account knows the next step is to run tapir, not "something is broken". */
+.empty-connected { border-style: solid; border-color: var(--accent); background: var(--accent-weak); color: var(--fg); }
+.empty-connected strong { color: var(--accent); }
 
 /* flash / notification banner */
 .flash { padding: var(--s2) var(--s3); border-radius: var(--radius); margin-bottom: var(--s4); font-size: .92rem; border: 1px solid var(--line); }