Add a vanilla-creature filter to card and library search

Filtering for creatures with an empty rules box can't be done from
`oracle_cards.oracle_text`: that column joins every face together, so an
Adventure card always carries its spell half's text and a naive `oracle_text
= ''` test would exclude exactly the cards worth including.

Derive it at ingest instead, from the front face's own text — the front is the
half that resolves as the creature, and adventure/prepared/transform cards are
all cast from it. Exposed as `vanilla_only` behind its own `has_vanilla`
capability check rather than folded into `has_universes_beyond`, since it
lands in a later reference-schema revision and a db can have one column
without the other. Needs a card-data re-download before it does anything.

src/api.ts

@@ -114,6 +114,8 @@ export interface LibraryFilter {
   legal_in?: string | null;
   /** Only cards that can head a Commander deck. */
   can_be_commander?: boolean;
+  /** Only creatures with an empty rules box (Adventure/Prepared creature halves count). */
+  vanilla_only?: boolean;
   /** Only cards printed in this set (matched against printings.set_code).
    *  Lets the release browser reuse the whole criteria pipeline. */
   set_code?: string | null;