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 {