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/channel_errors.go

@@ -0,0 +1,59 @@
+package store
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// ChannelError is a channel that returned HTTP 404 on a discovery pass.
+type ChannelError struct {
+	ChannelID   string
+	ChannelName string
+	FirstSeen   time.Time
+	LastSeen    time.Time
+}
+
+// UpsertChannelError records (or refreshes) a 404 channel for the current user.
+// Called by the runner inside a withUser scope; RLS guards user isolation.
+func (s *Store) UpsertChannelError(ctx context.Context, userID, channelID, channelName string) error {
+	return s.withUser(ctx, userID, func(tx pgx.Tx) error {
+		_, err := tx.Exec(ctx, `
+			INSERT INTO channel_errors (user_id, channel_id, channel_name)
+			VALUES ($1, $2, $3)
+			ON CONFLICT (user_id, channel_id)
+			DO UPDATE SET channel_name = EXCLUDED.channel_name, last_seen = now()`,
+			userID, channelID, channelName)
+		if err != nil {
+			return fmt.Errorf("store: upsert channel error: %w", err)
+		}
+		return nil
+	})
+}
+
+// ListChannelErrors returns all 404-flagged channels for the user, newest first.
+func (s *Store) ListChannelErrors(ctx context.Context, userID string) ([]ChannelError, error) {
+	var out []ChannelError
+	err := s.withUser(ctx, userID, func(tx pgx.Tx) error {
+		rows, err := tx.Query(ctx, `
+			SELECT channel_id, channel_name, first_seen, last_seen
+			FROM channel_errors
+			WHERE user_id = $1
+			ORDER BY last_seen DESC`, userID)
+		if err != nil {
+			return fmt.Errorf("store: list channel errors: %w", err)
+		}
+		defer rows.Close()
+		for rows.Next() {
+			var ce ChannelError
+			if err := rows.Scan(&ce.ChannelID, &ce.ChannelName, &ce.FirstSeen, &ce.LastSeen); err != nil {
+				return fmt.Errorf("store: scan channel error: %w", err)
+			}
+			out = append(out, ce)
+		}
+		return rows.Err()
+	})
+	return out, err
+}

internal/adapters/store/migrate_test.go

@@ -53,7 +53,9 @@ func TestMigration010LoginEventsUpDown(t *testing.T) {
 	require.True(t, loginEventsExists(t), "login_events must exist at latest migration")
 
 	m := fileMigrator(t)
-	// 011 and 012 sit above 010; step them down first so 010 is exercised in isolation.
+	// 011, 012, 013 sit above 010; step them down first so 010 is exercised in isolation.
+	require.NoError(t, m.Steps(-1), "down 013 drops channel_errors, login_events intact")
+	require.True(t, loginEventsExists(t), "013 down leaves login_events intact")
 	require.NoError(t, m.Steps(-1), "down 012 is a no-op, login_events intact")
 	require.True(t, loginEventsExists(t), "012 down leaves login_events intact")
 	require.NoError(t, m.Steps(-1), "down 011 must not touch login_events")
@@ -62,7 +64,7 @@ func TestMigration010LoginEventsUpDown(t *testing.T) {
 	require.NoError(t, m.Steps(-1), "down 010 must drop login_events")
 	require.False(t, loginEventsExists(t), "login_events must be gone after the down migration")
 
-	require.NoError(t, m.Steps(3), "up must recreate 010 then re-apply 011 and 012")
+	require.NoError(t, m.Steps(4), "up must recreate 010 then re-apply 011, 012, 013")
 	require.True(t, loginEventsExists(t), "login_events must be restored after the up migration")
 }
 
@@ -81,10 +83,11 @@ func autoSummarizeDefault(t *testing.T) string {
 // up sets the auto_summarize column default to TRUE (ADR-018), down restores
 // FALSE. The down intentionally does not revert existing rows — only the default.
 func TestMigration011AutoSummarizeDefaultUpDown(t *testing.T) {
-	newStore(t) // latest (012 applied)
+	newStore(t) // latest (013 applied)
 	require.Equal(t, "true", autoSummarizeDefault(t), "011 sets the default to TRUE")
 
 	m := fileMigrator(t)
+	require.NoError(t, m.Steps(-1), "down 013 drops channel_errors")
 	require.NoError(t, m.Steps(-1), "down 012 is a no-op")
 	require.NoError(t, m.Steps(-1), "down 011 reverts the column default")
 	require.Equal(t, "false", autoSummarizeDefault(t), "default is FALSE after the down migration")
@@ -92,6 +95,7 @@ func TestMigration011AutoSummarizeDefaultUpDown(t *testing.T) {
 	require.NoError(t, m.Steps(1), "up 011 re-applies the TRUE default")
 	require.Equal(t, "true", autoSummarizeDefault(t))
 	require.NoError(t, m.Steps(1), "up 012 runs clean (no FORCE RLS on fresh schema)")
+	require.NoError(t, m.Steps(1), "up 013 creates channel_errors")
 }
 
 // TestMigration012FixAutoSummarizeRLS proves 012 runs cleanly and flips any

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);