Printing tracking for owned copies and deck art, and an outdated-card-data notice

Owned copies can now be pinned to a printing + finish (nonfoil / foil /
etched), activating the long-dormant library_card_copies table as groups
of identical copies. library_cards.quantity_owned stays authoritative:
groups are a subset of it, and the rest are "unspecified". Tracking more
copies than are unspecified grows ownership to fit; lowering ownership
trims the most recently added groups first. Migration 0006 adds the
group quantity, a unique (card, printing, finish) index, and
deck_cards.preferred_scryfall_id.

A deck slot can show a chosen printing's art, both faces from that
printing, falling back to the default when the id no longer resolves.
It survives quantity/board changes and is kept by duplicate_deck.

UI: the click preview's info tile gets a Printings section when opened
from the Library or a deck (copy groups with steppers, "Add a
printing…", and "Shown in this deck" with Change…/Default), backed by a
printing picker grid. The picker takes Escape in the capture phase so
it closes without closing the preview under it.

Sync: CardEntry.printings (keyed "<scryfall_id>/<finish>") and
DeckCardEntry.printing. Cards now merge field by field with groups per
key, so a quantity edit and a printing added on another device both
survive; merged groups are clamped deterministically. A document is
written as version 2 only once it holds printing data, so older builds
and servers refuse it instead of dropping the data and syncing the loss.

Decklists: "(SET) NUM", optionally "*F*"/"*E*", round-trips through deck
and library export/import. Library export writes a line per group plus
one for unspecified copies.

reference.db printings gain lang and released_at for picker labels and
ordering. When the installed card data lacks any column this build's
reference schema has (found by diffing against the schema applied
in-memory, so no list to maintain), a notice under the top bar offers
Update card data (and Import card data… on phones); "Not now" is
remembered until something more goes missing.

README.md

@@ -205,6 +205,39 @@ Marvel Commander — isn't hidden.
 > once to enable them; until then they're inert rather than broken. Price,
 > format legality and *can be a commander* work immediately.
 
+### Printings
+
+The library counts cards, not printings, but you can say which printing a copy
+is when it matters. Click a card's image in the **Library** or a deck to open
+its preview. The **Owned** section lists your copies by printing and finish
+(nonfoil / foil / etched), with a stepper for each, and **Add a printing…**
+picks another. Copies you haven't assigned a printing to stay *unspecified*:
+"5 copies · 3 by printing · 2 unspecified". Assigning more copies to printings
+than you own raises the owned count to match. Lowering the owned count drops
+the most recently added printing entries first.
+
+In a deck, **Shown in this deck → Change…** picks which printing's art that
+slot shows, and **Default** goes back to the usual one. This is display only:
+it doesn't need to be a copy you own.
+
+Decklist export writes a chosen or tracked printing the way Moxfield and Arena
+do, and import reads it back:
+
+```text
+1 Sol Ring (2X2) 315 *F*
+4 Lightning Bolt (2XM) 129
+```
+
+`*F*` marks foil and `*E*` etched. A library export writes one line per
+printing, then a plain line for the unspecified copies. A suffix that doesn't
+match any printing still imports the card itself.
+
+> The picker's dates and languages need card data from this version. Until you
+> update it, printings are listed without them. Whenever your card data is
+> missing something the app could use, a notice under the top bar offers
+> **Update card data**, and **Import card data…** too on a phone. Dismissing it
+> with **Not now** lasts until a later version needs something more.
+
 ## Sync
 
 Your library always lives on the device you're using, so the app works the
@@ -235,6 +268,13 @@ either is lost) or **use the server's library** instead of this device's.
 The Info tab also lists the account's signed-in devices, so a lost phone can be
 signed out from another one.
 
+> **Printings and older versions.** A library with printings in it (see
+> [Printings](#printings)) is saved in a newer format. A device or server
+> running an older version refuses it and says to update, rather than syncing
+> and quietly losing them. Libraries that don't use printings stay readable by
+> older versions. Update the sync server before the first device starts using
+> printings.
+
 > **Upgrading from folder sync?** Earlier versions kept the library as a JSON
 > file in a folder your sync tool watched. Your library is already on each
 > device, so signing in to a server picks up from there; the first device to