fix: stop protecting stale, unverified pc-builder.io catalog prices

Bug found: "Gigabyte GAMING OC GeForce RTX 5070 Ti 16 GB Video Card" kept
showing a stale price of 431.17 even after pc-builder.io itself no longer
had a fixed offer for it (its "Add to Build" HTML only had a generic
Amazon search link, no .price-box element at all, confirmed by fetching
the live page directly).

Root cause: scrapeComponentType's "never lose a better price" rule (added
to make ScrapedPartsController#compare's persisted merchant prices
durable across catalog refreshes) was too broad. It treated ANY existing
price as worth protecting whenever the incoming catalog price was null or
higher - including a price that had only ever come from a *previous*
pc-builder.io scrape itself (priceSource: null), with no independent
verification. Since pc-builder.io's own listings change constantly (a
GPU can lose its priced Amazon link if the referenced ASIN goes out of
stock), that logic effectively froze old pc-builder.io prices forever
even after the site removed them.

Fix: extracted the decision into shouldKeepExistingPrice(), which now
only protects a price when priceSource is a real merchant name (set by
updatePriceIfCheaper after a live comparison confirmed it on the
merchant's own page) - never when priceSource is null (an unverified,
previous pc-builder.io catalog price). A plain re-scrape can always
correct/clear an unverified price, matching whatever pc-builder.io
currently shows.

4 new unit tests for shouldKeepExistingPrice covering: unverified
pc-builder.io price gets overwritten (the regression case), verified
merchant price is protected from worse/missing catalog data, verified
merchant price is still replaced by a genuinely cheaper catalog price,
and no-existing-part returns false.

Also re-ran `node ace scrape:parts` to apply the fix to already-cached
data with stale unverified prices (affected ~1500 rows across the
catalog before the fix, mostly likely unaffected in practice but no
longer silently trusted going forward).

tests/functional/scraped_parts.spec.ts

@@ -7,6 +7,7 @@ import {
   isCategoryStale,
   ensureFreshCategory,
   updatePriceIfCheaper,
+  shouldKeepExistingPrice,
   CACHE_TTL_HOURS,
 } from '#services/part_scraper_service'
 
@@ -215,3 +216,38 @@ test.group('Best-known-price persistence', () => {
     assert.equal(part.priceSource, 'Grosbill')
   })
 })
+
+test.group('Catalog refresh price protection (shouldKeepExistingPrice)', () => {
+  test('an unverified pc-builder.io price is NOT protected and gets overwritten', ({ assert }) => {
+    // Regression test: a GPU had price=431.17 from a previous pc-builder.io
+    // catalog scrape (priceSource: null, never independently verified).
+    // pc-builder.io later stopped showing a fixed price for it (only a
+    // generic Amazon search link). The stale 431.17 must NOT be kept.
+    const existing = { price: 431.17, priceSource: null }
+
+    assert.isFalse(shouldKeepExistingPrice(existing, null))
+    assert.isFalse(shouldKeepExistingPrice(existing, 999))
+  })
+
+  test('a verified merchant price IS protected from a worse/missing catalog price', ({
+    assert,
+  }) => {
+    const existing = { price: 350, priceSource: 'LDLC' }
+
+    assert.isTrue(shouldKeepExistingPrice(existing, null))
+    assert.isTrue(shouldKeepExistingPrice(existing, 500))
+  })
+
+  test('a verified merchant price is replaced if the catalog price is actually cheaper', ({
+    assert,
+  }) => {
+    const existing = { price: 350, priceSource: 'LDLC' }
+
+    assert.isFalse(shouldKeepExistingPrice(existing, 200))
+  })
+
+  test('no existing part means nothing to keep', ({ assert }) => {
+    assert.isFalse(shouldKeepExistingPrice(null, 100))
+    assert.isFalse(shouldKeepExistingPrice(null, null))
+  })
+})