Add GET /api/video-gen/history/:id so a video id resolves without downloading the whole history

[atomantic]

Summary

Gives video id → filename resolution a real home: GET /api/video-gen/history/:id.

A history entry's id is not its filename stem. videoGen/local.js names a clip <jobId>.mp4 (so path reconstruction happens to work for per-scene renders), but the video-timeline renderer — a Creative Director project's stitched final cut — mints timeline-<project>-<ts>.mp4 beside an independent randomUUID() id. A CD finalVideoId is therefore a history id, and /data/videos/<id>.mp4 404s for it. The thumbnail is keyed the other way (generateThumbnail always writes <jobId>.jpg), which is what made the mismatch easy to miss: the poster renders perfectly while the mp4 behind it does not exist.

With no by-id route, useVideoFileSrc resolved that by downloading the entire render history and scanning it client-side — on four surfaces (CD list cards, CD Overview, EpisodeVideoStage, MusicVideo).

• getHistoryItem(id) (server/services/videoGen/history.js) — the single-entry read, next to the existing loadHistory/saveHistory/mutateVideoHistory primitives. Returns null (not a throw) for an absent id so the route owns the 404, while the strict loadHistory read still throws on an unreadable history file rather than reporting a bogus "no such entry".
• GET /api/video-gen/history/:id — returns the entry, 404 NOT_FOUND for an unknown id.
• getVideoHistoryItem(id, options) (client/src/services/apiImageVideo.js) — the wrapper, and useVideoFileSrc now calls it instead of listVideoHistory. The hook's public shape ({ src, resolving, retry }), its synchronous resolving, its enabled laziness, and its silent-failure contract are all unchanged.

Two decisions the issue left open, recorded in full at #4165:

1. By-id endpoint, not a 302 resolver. Every consumer wants the entry, not just a redirect target; a resolver would also introduce a second URL space for the same bytes that the poster convention (/data/video-thumbnails/<jobId>.jpg) wouldn't follow.
2. The id is validated as a length-capped string, not with the existing UUID historyIdSchema. Ids this install mints are randomUUID(), but history entries also arrive with a caller-supplied download id (videoDownload.js: const jobId = id || randomUUID()) and from federated peers, so a .guid() gate would 400 rows that are legitimately in the list. Nothing is interpolated into a filesystem path — the value is only compared against stored ids.

ScenePreview keeps its <jobId>.mp4 reconstruction — deliberately. The issue notes the endpoint "would also let ScenePreview stop reconstructing paths at all". Its one remaining reconstructing caller is SegmentsTab, which renders a ScenePreview per treatment scene; those are per-scene clip renders where <jobId>.mp4 is correct by construction. Resolving internally would turn one render into N by-id requests on mount for a 10-scene treatment, buy nothing for those ids, and change autoplay timing inside a component shared by four surfaces. The reconstruction stays the documented fallback for callers that pass no src; every surface where the id genuinely is not the filename stem already passes a resolved src.

Test plan

• cd server && NODE_ENV=test npx vitest run routes/videoGen services/videoGen — 586 passed. New coverage:
  • server/services/videoGen/history.test.js — getHistoryItem resolves the timeline case (filename unrelated to id), returns null (not a throw, not undefined) for an absent id and an empty history, matches ids exactly rather than by filename stem, and survives a malformed row sitting ahead of the target.
  • server/routes/videoGen.test.js — the route returns the one entry and calls getHistoryItem with exactly the requested id while never touching loadHistory; 404s an unknown id; decodes a percent-encoded id before the lookup; 400s an over-length id without reaching the service.
• cd client && npx vitest run src/pages src/hooks src/components/pipeline src/services src/components/creative-director — 2226 passed. useVideoFileSrc.test.js now drives a by-id mock that rejects with a 404-shaped error for an unknown id (matching what request() really does), and adds a guard that the hook asks for exactly one id and never the whole list.
• cd client && npx biome lint --error-on-warnings on the touched files — clean.

Closes #4165