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/migrations/016_channel_caption_state.up.sql

@@ -0,0 +1,30 @@
+-- Migration 016: per-(user, channel) caption-availability memory (ADR-024).
+--
+-- Some channels never publish English captions (foreign-language news, music,
+-- etc.). Each of their new videos still costs ONE rate-limited caption fetch
+-- (ADR-014) before resolving to "none" — and on a throttled egress IP that fetch
+-- may 429 and churn through the backoff machinery first. This table remembers
+-- channels that repeatedly yield no captions so discovery can stop attempting
+-- their videos, freeing the scarce fetch budget for channels that do have them.
+--
+-- consecutive_none counts no-caption outcomes in a row; a successful fetch resets
+-- it to 0. Once it crosses the threshold the channel is suppressed until
+-- captionless_until, after which one video is re-probed (auto-recovery for a
+-- channel that starts adding captions). Per-user + RLS-scoped, consistent with
+-- the rest of the user-owned schema (subscriptions are per-user; ADR-012).
+CREATE TABLE channel_caption_state (
+    user_id           UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    channel_id        TEXT        NOT NULL,
+    consecutive_none  INT         NOT NULL DEFAULT 0,
+    captionless_until TIMESTAMPTZ,
+    updated_at        TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, channel_id)
+);
+
+CREATE INDEX idx_channel_caption_state_user_id ON channel_caption_state(user_id);
+
+ALTER TABLE channel_caption_state ENABLE ROW LEVEL SECURITY;
+ALTER TABLE channel_caption_state FORCE ROW LEVEL SECURITY;
+CREATE POLICY channel_caption_state_isolation ON channel_caption_state
+    FOR ALL
+    USING (user_id = current_setting('tapir.current_user_id', true)::uuid);