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/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,