Add feed/reader mode with full-article capture at ingest

Reader mode presents the stories on a page one at a time in a focused
overlay, marking each read as it's shown. Left/right arrows (or the header
book button / `f`) page through them; read stories dim on the grid. Read
state is device-local in localStorage.

Backing this required actually capturing article bodies, which Pete wasn't
doing — it kept only the RSS <description> lede and discarded content:encoded:

- stories.content column (idempotent migration; old rows fall back to lede)
- parser keeps content:encoded as paragraph-preserving text
- article fetch already done for paywall detection now also returns its body,
  so ingest stores the richer of feed-content vs scraped body with no extra
  request (prefers the archive snapshot body for paywalled stories)
- GET /api/article?id= serves the stored text; card queries now select id and
  expose it as data-id for the reader

Tests cover content extraction, the storage round-trip, and the article
endpoint + card rendering end to end.

internal/web/server.go

@@ -118,6 +118,7 @@ func New(cfg config.WebConfig, sources []config.SourceConfig) (*Server, error) {
 	mux.HandleFunc("GET /{$}", s.handleIndex)
 	mux.HandleFunc("GET /weather", s.handleWeatherDemo)
 	mux.HandleFunc("GET /api/search", s.handleSearch)
+	mux.HandleFunc("GET /api/article", s.handleArticle)
 	for _, ch := range channels {
 		ch := ch
 		mux.HandleFunc("GET /"+ch.Slug, func(w http.ResponseWriter, r *http.Request) {