Add card USD pricing and a deck maybeboard

Prices come from Scryfall's prices.usd on printings, shown as a hover badge
on card tiles. The price is picked independently of whichever printing
supplies the display image (the cheapest known price across all printings),
since the display printing is often a promo/variant with no listed price.

The maybeboard lets a deck track cards under consideration without counting
them in deck analysis stats or library assignment totals. Cards move between
mainboard and maybeboard via a per-card toggle; maybeboarding the commander
clears commander status.

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

crates/mtgc-core/src/db.rs

@@ -16,6 +16,7 @@ const LIBRARY_MIGRATIONS: &[&str] = &[
     include_str!("sql/library/0001_init.sql"),
     include_str!("sql/library/0002_location.sql"),
     include_str!("sql/library/0003_deck_format.sql"),
+    include_str!("sql/library/0004_deck_maybeboard.sql"),
 ];
 
 /// The attached alias under which the reference db is available at runtime.
@@ -34,6 +35,19 @@ pub fn image_subquery(column: &str) -> String {
     )
 }
 
+/// Correlated subquery selecting the cheapest known USD price across *all*
+/// printings of the oracle card aliased `oc` — deliberately independent of
+/// [`image_subquery`]'s printing choice, since the printing that happens to
+/// supply the display image (first by rowid) is often a promo/variant with no
+/// listed price even when other printings of the same card do have one.
+/// Returns NULL (read as 0.0) when no printing has a known price.
+pub fn price_subquery() -> String {
+    format!(
+        "(SELECT MIN(p.price_usd) FROM {REF_DB}.printings p
+          WHERE p.oracle_id = oc.oracle_id AND p.price_usd > 0)"
+    )
+}
+
 /// Create (if needed) and open a reference.db, applying the reference schema.
 /// Used by the ingester and by test fixtures.
 pub fn open_reference_for_build(path: &Path) -> Result<Connection> {