feat(discovery): per-channel caption-availability memory (ADR-024)

After transcript caching (ADR-021) and the Shorts filter (ADR-023), the remaining
caption waste is the first fetch on every new video of a channel that never has
English captions — each costs one rate-limited fetch to resolve to "none", and on
a throttled IP churns the backoff machinery first.

Remember, per (user, channel), a streak of consecutive no-caption outcomes
(channel_caption_state, migration 016, RLS-scoped). Once it reaches
TAPIR_CHANNEL_CAPTIONLESS_THRESHOLD (default 5) the channel is suppressed — videos
discovered/listed but not caption-fetched — for TAPIR_CHANNEL_CAPTIONLESS_WINDOW
(default 14d), then one is re-probed (auto-recovery). A successful fetch resets
the streak; a 429 does not count; an explicit manual request bypasses suppression.
threshold=0 disables.

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

internal/config/config.go

@@ -52,6 +52,14 @@ type Config struct {
 	// 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
+
+	// ChannelCaptionlessThreshold is how many consecutive no-caption results a
+	// channel may yield before its videos are suppressed from caption fetching
+	// (ADR-024). 0 disables the per-channel caption memory entirely.
+	ChannelCaptionlessThreshold int
+	// ChannelCaptionlessWindow is how long a suppressed channel stays suppressed
+	// before one video is re-probed (auto-recovery for a channel that adds captions).
+	ChannelCaptionlessWindow time.Duration
 	// SummarizerTimeout bounds a single completion call. Thinking models are
 	// slow, so the default is generous.
 	SummarizerTimeout time.Duration
@@ -145,6 +153,8 @@ const (
 	defaultSummaryMaxTokens      = 1500
 	defaultMaxTranscriptChars    = 18000
 	defaultMinVideoSeconds       = 60
+	defaultCaptionlessThreshold  = 5
+	defaultCaptionlessWindow     = 14 * 24 * time.Hour
 	defaultSummarizerTimeout     = 5 * time.Minute
 	defaultYTTokenRef            = "youtube/refresh_token"
 	defaultYTConnectRedirectURL  = "https://tapir.d-ma.be/oauth/youtube/callback"
@@ -246,6 +256,21 @@ func Load() (Config, error) {
 	}
 	c.MinVideoSeconds = minVideo
 
+	captionThreshold, err := intOr("TAPIR_CHANNEL_CAPTIONLESS_THRESHOLD", defaultCaptionlessThreshold)
+	if err != nil {
+		return Config{}, err
+	}
+	if captionThreshold < 0 {
+		captionThreshold = 0
+	}
+	c.ChannelCaptionlessThreshold = captionThreshold
+
+	captionWindow, err := durationOr("TAPIR_CHANNEL_CAPTIONLESS_WINDOW", defaultCaptionlessWindow)
+	if err != nil {
+		return Config{}, err
+	}
+	c.ChannelCaptionlessWindow = captionWindow
+
 	onboard, err := intOr("TAPIR_ONBOARD_SUMMARIZE_COUNT", defaultOnboardSummarizeCount)
 	if err != nil {
 		return Config{}, err