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/adapters/store/channel_caption_test.go

@@ -0,0 +1,67 @@
+package store_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestChannelCaptionMemory_SuppressesAfterThreshold(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	super := rawPool(t)
+	resetDB(t, super)
+	seedUser(t, super, userA)
+
+	const threshold = 3
+	window := time.Hour
+
+	// Below threshold: not yet suppressed.
+	require.NoError(t, s.RecordChannelCaptionOutcome(ctx, userA, "chanX", false, threshold, window))
+	require.NoError(t, s.RecordChannelCaptionOutcome(ctx, userA, "chanX", false, threshold, window))
+	got, err := s.CaptionlessChannels(ctx, userA)
+	require.NoError(t, err)
+	require.NotContains(t, got, "chanX", "2 < threshold 3: not suppressed yet")
+
+	// Crossing the threshold suppresses the channel.
+	require.NoError(t, s.RecordChannelCaptionOutcome(ctx, userA, "chanX", false, threshold, window))
+	got, err = s.CaptionlessChannels(ctx, userA)
+	require.NoError(t, err)
+	require.Contains(t, got, "chanX", "3 consecutive no-caption results suppress the channel")
+
+	// A successful caption fetch resets it.
+	require.NoError(t, s.RecordChannelCaptionOutcome(ctx, userA, "chanX", true, threshold, window))
+	got, err = s.CaptionlessChannels(ctx, userA)
+	require.NoError(t, err)
+	require.NotContains(t, got, "chanX", "a captioned video clears suppression")
+}
+
+func TestChannelCaptionMemory_WindowExpiryReProbes(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	super := rawPool(t)
+	resetDB(t, super)
+	seedUser(t, super, userA)
+
+	// A negative window means captionless_until lands in the past — modelling an
+	// elapsed suppression window, which must make the channel eligible again.
+	require.NoError(t, s.RecordChannelCaptionOutcome(ctx, userA, "chanY", false, 1, -time.Hour))
+	got, err := s.CaptionlessChannels(ctx, userA)
+	require.NoError(t, err)
+	require.NotContains(t, got, "chanY", "an expired window re-enables the channel for a re-probe")
+}
+
+func TestChannelCaptionMemory_DisabledThresholdIsNoOp(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	super := rawPool(t)
+	resetDB(t, super)
+	seedUser(t, super, userA)
+
+	require.NoError(t, s.RecordChannelCaptionOutcome(ctx, userA, "chanZ", false, 0, time.Hour))
+	got, err := s.CaptionlessChannels(ctx, userA)
+	require.NoError(t, err)
+	require.Empty(t, got, "threshold 0 disables the memory — nothing recorded")
+}