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.

crates/mtgc-core/src/model.rs

@@ -314,6 +314,12 @@ pub struct LibraryFilter {
     /// Only cards that can head a Commander deck.
     #[serde(default)]
     pub can_be_commander: bool,
+    /// Only "vanilla" creatures — a creature with an empty rules box. Includes
+    /// Adventure and Prepared cards whose creature half has no abilities, which
+    /// is why this reads a precomputed column instead of testing `oracle_text`:
+    /// that column joins every face, so the attached spell's text is in it.
+    #[serde(default)]
+    pub vanilla_only: bool,
     /// Only cards with an empty colour identity — what a colourless commander
     /// can actually play. The colour bitmasks can't express this: 0 there means
     /// "no filter", not "no colours".