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>

docs/homelab-integration.md

@@ -44,6 +44,11 @@ it** — endpoints and aliases drift, and this file is a snapshot (2026-06-06),
   the timedtext 429 path) and drops videos shorter than this plus any live/upcoming broadcast,
   so the scarce caption-fetch budget isn't spent on Shorts. `0` disables the filter. The
   paste-a-URL path is never filtered.
+- **Per-channel caption memory (ADR-024).** `TAPIR_CHANNEL_CAPTIONLESS_THRESHOLD` — **default
+  `5`** consecutive no-caption results before a channel is suppressed (its videos listed but not
+  caption-fetched). `TAPIR_CHANNEL_CAPTIONLESS_WINDOW` — **default `336h`** (14d) suppression
+  before one video is re-probed. `THRESHOLD=0` disables. A successful fetch resets the channel;
+  a 429 does not count; an explicit manual request bypasses suppression.
 - **Thinking models need an explicit `max_tokens`.** qwen3 / deepseek-r1 spend the budget on
   reasoning and return **empty content** if `max_tokens` is too low (or unset). The summarizer's
   parser treats an empty summary as an error for exactly this reason. **Done (2026-06-02, Worker F):**