feat(store,runner,web): channel unavailability notice (migration 013)

YouTube channels that 404 on playlist discovery (deleted/private) are now:
1. Wrapped in domain.ErrChannelUnavailable by the YouTube adapter (instead of
   a generic error), so the runner can identify them without string-matching.
2. Stored per-user in channel_errors (migration 013, RLS-guarded) via runner's
   new UpsertChannelError path — removed from the generic Errors counter,
   counted separately as ChannelUnavailable.
3. Shown on the account page under "Unavailable channels" with name, chip-warn
   badge, and first-seen date, so users know why some subscribed channels
   produce no videos.

internal/adapters/store/migrations/013_channel_errors.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS channel_errors;

internal/adapters/store/migrations/013_channel_errors.up.sql

@@ -0,0 +1,22 @@
+-- channel_errors: channels that returned HTTP 404 (deleted/private) on the most
+-- recent scheduler pass. Surfaced on the account page so users know why some
+-- subscribed channels produce no videos. Upserted per-pass; cleared when the
+-- channel starts returning results again (runner calls UpsertChannelError only
+-- on 404, so a recovered channel simply stops appearing after its row ages out
+-- or the user takes action). ON DELETE CASCADE keeps rows tidy on account deletion.
+CREATE TABLE channel_errors (
+    user_id      UUID        NOT NULL REFERENCES users(id)  ON DELETE CASCADE,
+    channel_id   TEXT        NOT NULL,
+    channel_name TEXT        NOT NULL,
+    first_seen   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    last_seen    TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, channel_id)
+);
+
+CREATE INDEX idx_channel_errors_user_id ON channel_errors(user_id);
+
+ALTER TABLE channel_errors ENABLE ROW LEVEL SECURITY;
+ALTER TABLE channel_errors FORCE ROW LEVEL SECURITY;
+CREATE POLICY channel_errors_isolation ON channel_errors
+    FOR ALL
+    USING (user_id = current_setting('tapir.current_user_id', true)::uuid);