feat(discovery): drop Shorts and livestreams before the caption fetch (ADR-023)

The scarce resource is the per-IP timedtext caption fetch (ADR-014); the pilot's
candidate set was mostly Shorts/clips/livestreams, each burning a fetch (a "none"
result is a completed fetch — it costs budget even when it yields nothing).

NewVideos now enriches candidates with one cheap Data API videos.list call
(contentDetails.duration + snippet.liveBroadcastContent — the quota API, a
DIFFERENT limit from the timedtext 429) and drops, before returning: videos
shorter than TAPIR_MIN_VIDEO_SECONDS (default 60) and any live/upcoming
broadcast. Dropped videos are never persisted, so the list declutters too.

Degrade-open: MinVideoSeconds=0 disables it (no quota call); a videos.list error
returns candidates unfiltered so discovery never breaks on a metadata hiccup. The
paste-a-URL path (VideoByID) is not filtered — an explicit request is honoured.

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

cmd/tapir/scheduler.go

@@ -33,6 +33,7 @@ func buildUserRunner(cfg config.Config, st *store.Store, secretStore ports.Secre
 		ClientSecret:       cfg.YTClientSecret,
 		TokenSecretRef:     web.YouTubeTokenRef(userID),
 		PreferredLanguages: []string{"en"},
+		MinVideoSeconds:    cfg.MinVideoSeconds,
 	}, secretStore)
 
 	engine := usecase.NewEngine(src, buildSummarizer(cfg), st)