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.

app/controllers/scraped_parts_controller.ts

@@ -2,7 +2,11 @@ import type { HttpContext } from '@adonisjs/core/http'
 import vine from '@vinejs/vine'
 import ScrapedPart from '#models/scraped_part'
 import ComponentType from '#models/component_type'
-import { ensureFreshCategory, updatePriceIfCheaper } from '#services/part_scraper_service'
+import {
+  ensureFreshCategory,
+  updatePriceIfCheaper,
+  clearPriceIfNoOffers,
+} from '#services/part_scraper_service'
 import { findCheapestOffers } from '#services/merchant_price_service'
 
 const SEARCH_RESULTS_LIMIT = 150
@@ -76,6 +80,11 @@ export default class ScrapedPartsController {
    * (or no price was known yet), it's persisted as the new default price
    * (see updatePriceIfCheaper) — so this comparison benefits every future
    * search/import for this part, not just the current user.
+   *
+   * If NO merchant confirms a price at all, the part's price is cleared
+   * to null (see clearPriceIfNoOffers) rather than silently keeping a
+   * pc-builder.io catalog price the user just explicitly asked to verify
+   * and that no longer checks out anywhere.
    */
   async compare({ request, auth, response }: HttpContext) {
     await auth.authenticate()
@@ -91,8 +100,10 @@ export default class ScrapedPartsController {
 
     if (offers.length > 0) {
       await updatePriceIfCheaper(part, offers[0])
+    } else {
+      await clearPriceIfNoOffers(part)
     }
 
-    return response.json({ offers })
+    return response.json({ offers, price: part.price })
   }
 }