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>

internal/config/config.go

@@ -46,6 +46,12 @@ type Config struct {
 	// MaxTranscriptChars bounds the transcript text sent to the model so a long
 	// transcript does not overflow a small-context primary. 0 disables truncation.
 	MaxTranscriptChars int
+
+	// MinVideoSeconds drops videos shorter than this from discovery (Shorts and
+	// other sub-minute clips that are noise and waste the scarce caption-fetch
+	// budget, ADR-014/ADR-023). Enforced via a cheap Data API videos.list lookup at
+	// discovery, never the rate-limited caption path. 0 disables the filter.
+	MinVideoSeconds int
 	// SummarizerTimeout bounds a single completion call. Thinking models are
 	// slow, so the default is generous.
 	SummarizerTimeout time.Duration
@@ -138,6 +144,7 @@ const (
 	defaultCloudFallbackModel    = "berget/mistral-small"
 	defaultSummaryMaxTokens      = 1500
 	defaultMaxTranscriptChars    = 18000
+	defaultMinVideoSeconds       = 60
 	defaultSummarizerTimeout     = 5 * time.Minute
 	defaultYTTokenRef            = "youtube/refresh_token"
 	defaultYTConnectRedirectURL  = "https://tapir.d-ma.be/oauth/youtube/callback"
@@ -230,6 +237,15 @@ func Load() (Config, error) {
 	}
 	c.MaxTranscriptChars = maxChars
 
+	minVideo, err := intOr("TAPIR_MIN_VIDEO_SECONDS", defaultMinVideoSeconds)
+	if err != nil {
+		return Config{}, err
+	}
+	if minVideo < 0 {
+		minVideo = 0
+	}
+	c.MinVideoSeconds = minVideo
+
 	onboard, err := intOr("TAPIR_ONBOARD_SUMMARIZE_COUNT", defaultOnboardSummarizeCount)
 	if err != nil {
 		return Config{}, err