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/view.go

@@ -2,6 +2,7 @@ package web
 
 import (
 	"regexp"
+	"slices"
 	"strings"
 	"time"
 	"unicode/utf8"
@@ -469,7 +470,7 @@ func (b listBuckets) empty() bool {
 // Dates are kept as the raw YYYY-MM-DD strings so the form re-renders the user's
 // input verbatim; parsing happens in matchFilter.
 type Filter struct {
-	Channel        string
+	Channels       []string // selected channel titles; empty = all channels
 	From           string
 	To             string
 	OnlySummarized bool // show only videos that have a summary
@@ -480,7 +481,28 @@ type Filter struct {
 // filter) the bar is hidden so the connect CTA stands alone (UX review C1); a
 // filter that happens to match nothing still shows the bar so it can be cleared.
 func (f Filter) active() bool {
-	return f.Channel != "" || f.From != "" || f.To != "" || f.OnlySummarized
+	return len(f.Channels) > 0 || f.From != "" || f.To != "" || f.OnlySummarized
+}
+
+// HasChannel reports whether a channel is currently selected (drives the
+// multi-select's selected state in the view).
+func (f Filter) HasChannel(c string) bool {
+	return slices.Contains(f.Channels, c)
+}
+
+// nonEmptyStrings drops blank entries. A channel multi-select submits real
+// channel titles; this guards against a stray empty value reaching the filter.
+func nonEmptyStrings(ss []string) []string {
+	out := ss[:0:0]
+	for _, s := range ss {
+		if strings.TrimSpace(s) != "" {
+			out = append(out, s)
+		}
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	return out
 }
 
 // matches reports whether a row satisfies the filter. Channel is an exact match;
@@ -491,7 +513,7 @@ func (f Filter) matches(r store.SummaryRow) bool {
 	if f.OnlySummarized && !r.Summarized {
 		return false
 	}
-	if f.Channel != "" && r.Channel != f.Channel {
+	if len(f.Channels) > 0 && !slices.Contains(f.Channels, r.ChannelTitle) {
 		return false
 	}
 	if from, ok := parseDate(f.From); ok {
@@ -521,7 +543,7 @@ func parseDate(s string) (time.Time, bool) {
 
 // apply returns the subset of rows matching the filter, preserving order.
 func (f Filter) apply(rows []store.SummaryRow) []store.SummaryRow {
-	if f.Channel == "" && f.From == "" && f.To == "" && !f.OnlySummarized {
+	if len(f.Channels) == 0 && f.From == "" && f.To == "" && !f.OnlySummarized {
 		return rows
 	}
 	out := rows[:0:0]