feat(web): real channel filter — multi-select of the user's channels

The free-text 'channel' filter was dead: it exact-matched SummaryRow.Channel,
which is just the provider ('youtube'), because videos never stored their source
channel. Now they do.

- migration 014: videos.channel_title (nullable; existing rows backfill on the
  next discovery pass, pasted videos immediately).
- discovery (NewVideos) + paste (VideoByID) populate channel_title; UpsertVideo
  persists it, preserving an existing title when an update arrives empty.
- store.DistinctChannels lists a user's channels (RLS-scoped); SummaryRow carries
  ChannelTitle via the shared projection.
- Filter: single Channel -> Channels []string, matching on ChannelTitle; the feed
  renders a multi-select of DistinctChannels (hidden until channels exist).
- migrate tests: 014 reversibility + fixed the relative-step counts in the 010/011
  up/down tests (014 shifted the topology).

TDD throughout: channel persist + distinct, adapter channel wiring, multi-channel
filter match, handler channel filter, migration up/down.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/views.templ

@@ -104,14 +104,14 @@ templ flashBanner(code string) {
 // #summary-list region; a non-HTMX request renders the whole page. flash carries
 // a one-shot notification (e.g. "connected", "registered") surfaced on arrival
 // after a POST→redirect.
-templ ListPage(b listBuckets, f Filter, stats PipelineStats, flash string, hasConnected bool) {
+templ ListPage(b listBuckets, f Filter, stats PipelineStats, flash string, hasConnected bool, channels []string) {
 	@Layout("Tapir — Summaries") {
 		@flashBanner(flash)
 		if hasConnected {
 			@pasteForm()
 		}
 		if !b.empty() || f.active() {
-			@filterForm(f)
+			@filterForm(f, channels)
 		}
 		if stats.RateLimited > 0 || stats.Pending > 0 || stats.NoText > 0 {
 			@pipelineBar(stats)
@@ -168,7 +168,7 @@ templ pasteForm() {
 	<div id="paste-result"></div>
 }
 
-templ filterForm(f Filter) {
+templ filterForm(f Filter, channels []string) {
 	<form
 		class="filters"
 		method="get"
@@ -178,7 +178,16 @@ templ filterForm(f Filter) {
 		hx-swap="innerHTML"
 		hx-indicator="#filter-indicator"
 	>
-		<label>Channel <input type="text" name="channel" value={ f.Channel } placeholder="any"/></label>
+		if len(channels) > 0 {
+			<label>
+				Channels
+				<select name="channel" multiple size="4">
+					for _, c := range channels {
+						<option value={ c } selected?={ f.HasChannel(c) }>{ c }</option>
+					}
+				</select>
+			</label>
+		}
 		<label class="filter-check">
 			<input type="checkbox" name="summarized" value="1" if f.OnlySummarized { checked }/>
 			Summarized only