Precompute content_chars to drop per-render body scans

The N-min-read chip derived reading time via LENGTH(content) over the
full article-body TEXT column on every listing render. LENGTH can't use
an index, so SQLite read each row's whole body per request on the hottest
path. Cache the character count in a content_chars column filled at insert
time (backfilled for existing rows), and point StoryContentLengths at it.

internal/storage/db.go

@@ -85,6 +85,11 @@ func runMigrations(d *sql.DB) error {
 	// else the body scraped during paywall detection) for reader mode. Stories
 	// ingested before this column existed simply have NULL and fall back to lede.
 	addColumnIfMissing(d, "stories", "content", "TEXT")
+	// content_chars caches the character count of content so the "N min read"
+	// chip never has to LENGTH() the full body on the hot listing path. Filled at
+	// insert time; the backfill below populates rows that predate the column.
+	addColumnIfMissing(d, "stories", "content_chars", "INTEGER NOT NULL DEFAULT 0")
+	backfillContentChars(d)
 	addColumnIfMissing(d, "stories", "published_at", "INTEGER")
 	addColumnIfMissing(d, "post_log", "url_canonical", "TEXT")
 	addColumnIfMissing(d, "post_log", "forced", "INTEGER NOT NULL DEFAULT 0")
@@ -154,6 +159,20 @@ func exec(label, query string, args ...any) {
 	}
 }
 
+// backfillContentChars populates content_chars for rows carrying a body but a
+// zero count — i.e. stories ingested before the column existed. LENGTH() counts
+// characters (code points) for TEXT, matching the utf8.RuneCountInString done at
+// insert. After the first run this matches no rows (bodied stories are set,
+// bodyless ones stay 0 and are filtered by content IS NOT NULL), so it's a cheap
+// startup no-op thereafter.
+func backfillContentChars(d *sql.DB) {
+	if _, err := d.Exec(
+		`UPDATE stories SET content_chars = LENGTH(content)
+		 WHERE content_chars = 0 AND content IS NOT NULL AND content <> ''`); err != nil {
+		slog.Error("backfill content_chars failed", "err", err)
+	}
+}
+
 func addColumnIfMissing(d *sql.DB, table, column, columnType string) {
 	q := fmt.Sprintf("ALTER TABLE %s ADD COLUMN %s %s", table, column, columnType)
 	if _, err := d.Exec(q); err != nil {

internal/storage/queries.go

@@ -7,6 +7,7 @@ import (
 	"log/slog"
 	"strings"
 	"time"
+	"unicode/utf8"
 )
 
 func nowUnix() int64 {
@@ -38,9 +39,9 @@ func InsertStory(s *Story) error {
 		publishedAt = s.PublishedAt
 	}
 	_, err := Get().Exec(
-		`INSERT INTO stories (guid, headline, lede, content, image_url, article_url, url_canonical, headline_norm, source, platforms, channel, classified, paywalled, seen_at, published_at)
-		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
-		s.GUID, s.Headline, s.Lede, nullIfEmpty(s.Content), s.ImageURL, s.ArticleURL, nullIfEmpty(s.URLCanonical), nullIfEmpty(s.HeadlineNorm), s.Source, s.Platforms, s.Channel, classified, paywalled, s.SeenAt, publishedAt,
+		`INSERT INTO stories (guid, headline, lede, content, content_chars, image_url, article_url, url_canonical, headline_norm, source, platforms, channel, classified, paywalled, seen_at, published_at)
+		 VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+		s.GUID, s.Headline, s.Lede, nullIfEmpty(s.Content), utf8.RuneCountInString(s.Content), s.ImageURL, s.ArticleURL, nullIfEmpty(s.URLCanonical), nullIfEmpty(s.HeadlineNorm), s.Source, s.Platforms, s.Channel, classified, paywalled, s.SeenAt, publishedAt,
 	)
 	return err
 }
@@ -414,8 +415,10 @@ func TrendingStories(limit int, sinceDay int64) ([]Story, error) {
 
 // StoryContentLengths returns the captured article text length (in characters)
 // for the given story ids, as a map keyed by id. Used to estimate a "N min read"
-// chip without transferring the full body: only LENGTH(content) crosses the
-// wire. Ids with no captured content are absent (treated as zero by callers).
+// chip. It reads the precomputed content_chars column rather than LENGTH()-ing
+// the full body, so the hot listing path never scans article text. Ids with no
+// captured content have content_chars = 0 and are absent from the map (treated
+// as zero by callers).
 func StoryContentLengths(ids []int64) map[int64]int {
 	out := make(map[int64]int, len(ids))
 	if len(ids) == 0 {
@@ -423,8 +426,8 @@ func StoryContentLengths(ids []int64) map[int64]int {
 	}
 	ph, args := intInClause(ids)
 	rows, err := Get().Query(
-		`SELECT id, LENGTH(content) FROM stories
-		 WHERE id IN (`+ph+`) AND content IS NOT NULL AND content <> ''`, args...)
+		`SELECT id, content_chars FROM stories
+		 WHERE id IN (`+ph+`) AND content_chars > 0`, args...)
 	if err != nil {
 		slog.Error("story content lengths query failed", "err", err)
 		return out

internal/storage/schema.go

@@ -7,6 +7,7 @@ CREATE TABLE IF NOT EXISTS stories (
     headline      TEXT NOT NULL,
     lede          TEXT,
     content       TEXT,
+    content_chars INTEGER NOT NULL DEFAULT 0,
     image_url     TEXT,
     article_url   TEXT NOT NULL,
     url_canonical TEXT,