feat(apicore-sync): field-level ownership between ApiCore and the shop

ApiCore stays the source of truth for beekeeping facts, Wabenhain for
everything sales-related; both remain separate packages talking only over
ApiCore's HTTP API.

- Batches: lot number, fill date, best-before date and filled jar count
  now follow ApiCore on every run (before: created once, corrections never
  arrived). remainingGlasses - the shop's own stock - is never written; if
  it exceeds a corrected fill quantity, the run reports it instead of
  silently adjusting.
- Batches are matched by the new Batch.apicoreBottlingId instead of the lot
  number, so correcting a lot number in ApiCore no longer creates a second
  batch. Existing synced batches are adopted once via their lot number.
- Harvests: date and year always follow ApiCore, also when published or
  edited; type, location and text stay as maintained in the admin.
- Products stay entirely shop-owned after creation (weight drives the
  legally required unit price, so it is never changed automatically).
- Admin panel: batch update count and stock-conflict notice; intro text
  states which system owns which fields.

Migration is additive (nullable unique column).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

prisma/schema.prisma

@@ -178,6 +178,10 @@ model Batch {
   id               String   @id @default(cuid())
   /// z.B. FH-26-01
   code             String   @unique
+  /// ApiCore-ID der Abfuellung (BottlingBatch), aus der diese Charge stammt.
+  /// Stabiler Zuordnungsschluessel fuer den Sync: die Losnummer (code) kann in
+  /// ApiCore korrigiert werden, die ID nicht. Null bei manuell angelegten Chargen.
+  apicoreBottlingId String?  @unique
   harvestId        String
   fillDate         DateTime
   bestBeforeDate   DateTime