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/web/views.templ

@@ -1,6 +1,7 @@
 package web
 
 import (
+	"fmt"
 	"strings"
 
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/store"
@@ -397,7 +398,7 @@ templ InviteNoticePage(message string, showLogin bool) {
 // signed-in email, the user's connected video accounts (each with a Disconnect
 // control), a Connect-YouTube link when none is connected, and the delete-account
 // danger zone. flash surfaces a one-shot notification (disconnect/connect).
-templ AccountPage(displayName, email string, conns []store.Connection, autoSummarize bool, flash string) {
+templ AccountPage(displayName, email string, conns []store.Connection, autoSummarize bool, channelErrors []store.ChannelError, flash string) {
 	@Layout("Tapir — Account") {
 		@flashBanner(flash)
 		<article class="account">
@@ -419,6 +420,24 @@ templ AccountPage(displayName, email string, conns []store.Connection, autoSumma
 				</p>
 				@summarizeModeControl(autoSummarize)
 			</section>
+			if len(channelErrors) > 0 {
+				<section class="channel-errors">
+					<h2>Unavailable channels</h2>
+					<p class="muted">
+						{ fmt.Sprintf("%d channel(s) returned errors on the last discovery pass.", len(channelErrors)) }
+						These may have been deleted or made private on YouTube.
+					</p>
+					<ul class="channel-error-list">
+						for _, ce := range channelErrors {
+							<li>
+								<span class="channel-error-name">{ ce.ChannelName }</span>
+								<span class="chip chip-warn">unavailable</span>
+								<span class="muted channel-error-since">since { ce.FirstSeen.Format("2006-01-02") }</span>
+							</li>
+						}
+					</ul>
+				</section>
+			}
 			<section>
 				<h2>Connected accounts</h2>
 				if len(conns) == 0 {