fix: clear price to null when a comparison finds no merchant offer

Several memory sticks (e.g. Crucial CT8G48C40U5, Patriot Signature Line,
Kingston FURY Beast) kept displaying their pc-builder.io catalog price
("€35.99", "€39.99"...) even after clicking "Compare prices" and getting
"No live price found, using catalog price" - an unconfirmed number was
being shown as if it were still valid, which is misleading given
pc-builder.io's catalog is known to go stale (see the earlier RTX 5070
€128 vs €66.89 case).

New clearPriceIfNoOffers(): when ScrapedPartsController#compare finds
zero offers across all 7 supported merchants, the part's price/priceSource
are reset to null, regardless of whether the price previously came from
the pc-builder.io catalog or an earlier verified merchant match - per
explicit product decision, an empty comparison always wins over keeping a
possibly-stale number around.

- ScrapedPartsController#compare now calls clearPriceIfNoOffers() in the
  empty-offers branch, and returns the resulting price in the JSON
  response so the frontend can mirror it exactly
- NewPartSearch.vue: comparePrices() now sets part.price from the
  response's `price` field (mirroring exactly what the backend just
  persisted) instead of only updating it on a cheaper find; removed the
  now-inaccurate "using catalog price €X" message since that price no
  longer exists once compared with no result
- 3 new tests for clearPriceIfNoOffers: clears an unverified
  pc-builder.io price, also clears a previously-verified merchant price
  when no longer confirmed, and is a no-op when there was no price to
  begin with

Verified end-to-end against the reported Crucial CT8G48C40U5 example:
price went from 35.99 (pc-builder.io) to null after a real comparison
found zero matching offers on any merchant.

tests/functional/scraped_parts.spec.ts

@@ -7,6 +7,7 @@ import {
   isCategoryStale,
   ensureFreshCategory,
   updatePriceIfCheaper,
+  clearPriceIfNoOffers,
   shouldKeepExistingPrice,
   CACHE_TTL_HOURS,
 } from '#services/part_scraper_service'
@@ -217,6 +218,53 @@ test.group('Best-known-price persistence', () => {
   })
 })
 
+test.group('clearPriceIfNoOffers', () => {
+  test('clears an unverified pc-builder.io catalog price when no merchant confirms it', async ({
+    assert,
+  }) => {
+    // Regression: memory sticks kept showing pc-builder.io catalog prices
+    // (e.g. €35.99) even though comparing prices found no matching offer
+    // on any merchant ("No live price found, using catalog price €35.99")
+    // — an unconfirmed number should not be displayed as if it were still
+    // valid.
+    const type = await createType('Memory-clear-1')
+    const part = await createScrapedPart(type.id, 'ZZZ-Test-Clear-A', 'clear-a-p1', 35.99)
+
+    await clearPriceIfNoOffers(part)
+    await part.refresh()
+
+    assert.isNull(part.price)
+    assert.isNull(part.priceSource)
+  })
+
+  test('also clears a price previously verified by a merchant if no longer confirmed', async ({
+    assert,
+  }) => {
+    const type = await createType('Memory-clear-2')
+    const part = await createScrapedPart(type.id, 'ZZZ-Test-Clear-B', 'clear-b-p1', 50)
+    part.priceSource = 'LDLC'
+    await part.save()
+
+    await clearPriceIfNoOffers(part)
+    await part.refresh()
+
+    assert.isNull(part.price)
+    assert.isNull(part.priceSource)
+  })
+
+  test('is a no-op when the part already has no price', async ({ assert }) => {
+    const type = await createType('Memory-clear-3')
+    const part = await createScrapedPart(type.id, 'ZZZ-Test-Clear-C', 'clear-c-p1', null)
+    const originalCheckedAt = part.priceCheckedAt
+
+    await clearPriceIfNoOffers(part)
+    await part.refresh()
+
+    assert.isNull(part.price)
+    assert.equal(part.priceCheckedAt, originalCheckedAt)
+  })
+})
+
 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