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/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