Prefer Scryfall's representative printing for default card images

Card previews were picking whichever printing happened to load first
during ingest (ORDER BY rowid over every printing, promos and foreign
versions included), so a card's default image was effectively random.
Capture the representative printing's images straight from the Oracle
Cards bulk pass instead, and prefer that over the old printings-table
guess, falling back gracefully on an older reference.db that predates
these columns.

Also add docs/printing-tracking-design.md, a design (not yet
implemented) for optionally tracking which specific printing/finish a
library or deck card is.

crates/mtgc-core/src/synergy.rs

@@ -317,8 +317,8 @@ impl Catalog {
              WHERE oc.oracle_id NOT IN (SELECT oracle_id FROM deck_cards WHERE deck_id = ?)
                AND (oc.color_identity & ?) = 0
                AND (",
-            image = crate::db::image_subquery("image_uri"),
-            back = crate::db::image_subquery("back_image_uri"),
+            image = self.image_expr("image_uri"),
+            back = self.image_expr("back_image_uri"),
             price = crate::db::price_subquery(),
             horizontal = self.horizontal_col(),
         );