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/domain/domain.go

@@ -2,7 +2,22 @@
 // the standard library — no providers, no storage, no AI. See docs/data-model.md.
 package domain
 
-import "time"
+import (
+	"fmt"
+	"time"
+)
+
+// ErrChannelUnavailable is returned by a VideoSource when a channel's upload
+// playlist returns HTTP 404 — the channel was deleted or made private. The runner
+// stores these so the account page can surface them to the user.
+type ErrChannelUnavailable struct {
+	ChannelID    string
+	ChannelTitle string
+}
+
+func (e *ErrChannelUnavailable) Error() string {
+	return fmt.Sprintf("channel %q (%s) unavailable: playlist not found", e.ChannelTitle, e.ChannelID)
+}
 
 // Provider identifies a video platform.
 type Provider string