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/sql/reference_schema.sql

@@ -46,7 +46,8 @@ CREATE TABLE IF NOT EXISTS printings (
     collector_number TEXT NOT NULL DEFAULT '',
     finishes         TEXT NOT NULL DEFAULT '',   -- comma-joined, e.g. "nonfoil,foil"
     rarity           TEXT NOT NULL DEFAULT '',
-    image_uri        TEXT NOT NULL DEFAULT ''
+    image_uri        TEXT NOT NULL DEFAULT '',
+    back_image_uri   TEXT NOT NULL DEFAULT ''   -- transform/MDFC/reversible back face
 );
 CREATE INDEX IF NOT EXISTS idx_printings_oid ON printings(oracle_id);