Add CSV price guides and PriceCharting alongside eBay

An eBay production keyset needs account verification, which leaves pricing
blocked on someone else's review queue. These are two routes that are not.

CSV price guide, POST /api/prices/import: no account, works immediately.
Column names are matched by alias, so a PriceCharting bulk export
(product-name / console-name / loose-price) and a hand-kept
title,system,loose,cib,new sheet both parse, along with currency symbols,
thousands separators and blank cells. Rows match on title + system, so the
same game on two consoles is priced separately, and rows for games not in
the library are reported rather than silently added.

PriceCharting adapter: paid, but access is immediate with no review, and it
quotes the same three tiers this app stores, so no inference is needed.
Their API docs are not reachable without an account, so the parser follows
the widely-used convention — integer pennies under hyphenated keys — and is
tolerant enough that a naming difference degrades to "no price" instead of
throwing. One method to adjust if it differs.

Providers are now a registry rather than a single service. /api/prices/status
lists each one with what it is configured for, what its numbers actually
mean, and how to enable it; refresh takes an optional provider name and
falls back to the first configured one. With none configured it answers 503
pointing at the CSV route.

Checked against the real library: a five-row guide in PriceCharting's own
column names priced four games and reported the fifth as not owned, with
each effective value following that copy's condition. Demo figures were
cleared afterwards.

135 backend tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.env.example

@@ -33,7 +33,20 @@ JWT_AUDIENCE=LudosData
 CORS_ORIGIN=http://localhost:8080
 
 # --- Market value (optional) ------------------------------------------------
-# Prices come from eBay's Browse API, which needs a free developer account.
+# Three routes, none of which block the others. Nothing here is required: with
+# all of it blank, prices can still be imported as a CSV or typed in by hand.
+#
+# 1. CSV PRICE GUIDE — no account, works immediately.
+#    POST a CSV to /api/prices/import. Columns are matched by name, so a
+#    PriceCharting bulk download, a spreadsheet you maintain, or any other list
+#    all work. Nothing to configure here.
+#
+# 2. PRICECHARTING — paid, but access is immediate with no review, which makes
+#    it the practical choice while an eBay keyset is in verification. Token
+#    comes from the Subscriptions page, "API/Download" button.
+PRICECHARTING_TOKEN=
+
+# 3. EBAY BROWSE — free, but the production keyset needs account verification.
 #
 #   1. Register at https://developer.ebay.com and create a developer account
 #   2. Create an application keyset (Application Keys -> Production)