fix(short): cerrar el bucle de edición y el desfase spec/vídeo en la subida

Dos huecos de la revisión:

1. /short_spec prometía re-renderizar el spec editado pero no existía el
   camino de vuelta. Ahora un .json adjunto se valida contra el contrato
   vivo (errores con su ruta verbatim), se re-comprueba el fundamento (la
   edición pudo meter una cifra nueva), se guarda como output nuevo y se
   renderiza. Sin LLM: este camino es gratis. La sesión sale del nombre
   del fichero (short_{id}_spec.json), que Telegram conserva al reenviar.

2. /upload_short podía subir un vídeo viejo con metadatos nuevos: produce
   guarda el spec ANTES de renderizar, así que un re-intento con render
   fallido deja en disco el MP4 de la vuelta anterior. Ahora se compara el
   mtime del vídeo con el created_at del spec y se niega (force lo salta).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -40,7 +40,7 @@ OutputGenerator (Ollama)
 | `/finish` | Stop early, proceed to generation |
 | `/generate podcast\|blog\|report\|thread` | Generate output |
 | `/generate short_en` | Vertical Short: shot spec → grounding check → MP4 |
-| `/short_spec` | Last shot spec as a JSON file, to hand-edit and re-render |
+| `/short_spec` | Last shot spec as a JSON file; edit it and send it back to re-render free |
 | `/upload_short` | Upload the rendered Short to YouTube (private, for review) |
 | `/sources` | List all sources found |
 | `/cancel` | Cancel current research |
@@ -84,6 +84,14 @@ Fallbacks hold throughout: if shortsmith is unreachable, the job errors, or the
 spec never validates, the spec JSON comes back as a file. The expensive part is
 the generation, not the render.
 
+**Hand-editing loop:** `/short_spec` hands you the spec as
+`short_{session_id}_spec.json`; edit it and send the file back to the bot. It
+validates against the live contract (errors come back with their exact paths),
+**re-runs the grounding check** — your edit may have introduced a new figure —
+saves the edited spec as a new output, and renders. No LLM in that path: it is
+free. The session comes from the filename, so it works even if the chat has
+researched something else since.
+
 Full spec of the phase: `docs/shortsmith-phase2-spec.md`.
 
 ## YouTube (`/upload_short`)
@@ -92,7 +100,9 @@ Uploads `/data/shorts/{session_id}.mp4` to the channel with the title from the
 spec, a description carrying the article link and the sources the Short cites on
 screen, and tags derived from the topic. The YouTube URL is written back to the
 output row, so a second `/upload_short` on the same session refuses unless you
-say `/upload_short force`.
+say `/upload_short force`. It also refuses if the MP4 on disk is **older than
+the latest saved spec** — that happens when a spec regeneration's render fails,
+and uploading would put the new metadata on the old video.
 
 **Read this before setting it up.** Videos uploaded through `videos.insert` from
 an **unaudited API project** are [restricted to private viewing