feat(listings): add source field to PriceHistory + unit tests

- Add `source` column to PriceHistory Prisma model (manual_update, admin_override, market_adjustment)
- Add migration for the new column with default 'manual_update'
- Update ListingPriceChangedEvent domain event with optional source parameter
- Update RecordPriceHistoryHandler to persist source
- Update GetPriceHistoryHandler to return source in query results
- Add unit tests for RecordPriceHistoryHandler (5 cases)
- Add unit tests for GetPriceHistoryHandler (3 cases)
- Add ListingPriceChangedEvent tests to domain events spec (4 cases)
- Add getPriceHistory controller tests (2 cases)

All 1805 tests pass, typecheck clean.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/schema.prisma

@@ -366,6 +366,7 @@ model PriceHistory {
   listing   Listing  @relation(fields: [listingId], references: [id], onDelete: Cascade)
   oldPrice  BigInt
   newPrice  BigInt
+  source    String   @default("manual_update")
   changedAt DateTime @default(now())
 
   @@index([listingId, changedAt(sort: Desc)])