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>

DECISIONS.md

@@ -891,6 +891,38 @@ this is well under the 10k/day cap; at larger scale, batch `videos.list` across
 
 ---
 
+## ADR-024 — Per-channel caption-availability memory
+
+**Status:** Accepted (2026-06-10). **Builds on ADR-014** (per-IP caption budget), **ADR-021**
+(shared transcript cache), **ADR-023** (Shorts filter).
+
+**Context.** After ADR-021 caches transcripts and ADR-023 drops Shorts, the remaining caption
+waste is the *first* fetch on every new video of a channel that never publishes English captions
+(foreign-language news, music, etc.). Each costs one rate-limited fetch to resolve to "none" —
+and on a throttled IP that fetch may 429 and churn the backoff machinery before it ever gets a
+verdict. A pilot user's feed had several such channels.
+
+**Decision.** Remember, per `(user, channel)`, a streak of consecutive no-caption outcomes
+(`channel_caption_state`, migration 016, RLS-scoped like the rest of the user-owned schema).
+Once the streak reaches `TAPIR_CHANNEL_CAPTIONLESS_THRESHOLD` (default 5) the channel is
+suppressed — its videos are discovered/listed but not caption-fetched — for
+`TAPIR_CHANNEL_CAPTIONLESS_WINDOW` (default 14d), after which one video is re-probed
+(auto-recovery for a channel that starts adding captions). A successful fetch resets the streak;
+a fresh 429 does NOT count (transient, not a caption verdict). An explicit manual request
+bypasses suppression. `threshold = 0` disables the feature.
+
+**Why per-user, not global.** Caption availability is really a channel property (public), so a
+global table would let users share the learning. But subscriptions are per-user (ADR-012) and at
+pilot scale users' channel sets barely overlap, so per-user + RLS keeps it consistent with the
+existing isolation model with no new non-RLS exception to justify. Promoting to a shared table
+(like transcripts, ADR-021) is a future optimisation if channel overlap grows.
+
+**Reversibility.** Migration 016 is a clean drop; `threshold = 0` disables at runtime. The
+memory only ever *suppresses fetches* — it never deletes content or affects already-stored
+summaries.
+
+---
+
 ## Rejected alternatives
 
 Approaches considered during the 2026-06-02 planning + grill session and **deliberately not