Capture back-face printing images for double-faced cards

Add a back_image_uri column to printings, populated during ingest from
a card's second card_faces image (only present for transform/MDFC/
reversible cards — split/adventure cards share one combined image and
correctly get ""). Plumb it through CardRef across every query path
(search, library, filters, deck contents) via the same printing row,
using a shared deterministic ORDER BY so front/back always come from
one printing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/mtgc-core/src/model.rs

@@ -88,6 +88,9 @@ pub struct CardRef {
     /// URL of a card image (Scryfall "normal" size) for one of its printings,
     /// or empty if none is available.
     pub image_uri: String,
+    /// URL of the *back face* image for the same printing (transform / modal
+    /// double-faced / reversible cards), or empty for single-faced cards.
+    pub back_image_uri: String,
 }
 
 /// A library row: a card the user owns, with quantity and its deck memberships.