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/library.rs

@@ -85,8 +85,8 @@ impl Catalog {
              JOIN {REF_DB}.oracle_cards oc ON oc.oracle_id = lc.oracle_id
              WHERE lc.quantity_owned > 0
              ORDER BY oc.name COLLATE NOCASE",
-            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()
         ))?;
@@ -314,6 +314,36 @@ mod tests {
         assert_eq!(stats.color_counts, [0, 0, 1, 0, 3]);
     }
 
+    #[test]
+    fn oracle_cards_image_wins_over_the_printings_pick() {
+        // The bug this guards: `printings` can hold an arbitrary (promo,
+        // showcase, foreign-language...) printing per card. `oracle_cards`'s
+        // own image — Scryfall's representative-printing pick, captured at
+        // ingest — must win when present, rather than whatever `printings`
+        // subquery would otherwise resolve to (first by rowid).
+        let fx = test_support::make();
+        let cat = &fx.catalog;
+        cat.connection()
+            .execute(
+                "UPDATE ref.printings SET image_uri = 'http://img/promo-variant' \
+                 WHERE oracle_id = 'sol-ring'",
+                [],
+            )
+            .unwrap();
+        cat.connection()
+            .execute(
+                "UPDATE ref.oracle_cards SET image_uri = 'http://img/normal' \
+                 WHERE oracle_id = 'sol-ring'",
+                [],
+            )
+            .unwrap();
+        cat.add_owned("sol-ring", 1).unwrap();
+
+        let lib = cat.list_library().unwrap();
+        let sol = lib.iter().find(|c| c.card.oracle_id == "sol-ring").unwrap();
+        assert_eq!(sol.card.image_uri, "http://img/normal");
+    }
+
     #[test]
     fn back_image_uri_is_plumbed_through() {
         let fx = test_support::make();