fix(harvests): allow several harvests of the same type per year

The ApiCore sync failed with "Unique constraint failed on the constraint:
harvests_year_type_key" as soon as ApiCore had two harvests of the same
honey type in one year, which is normal (several extractions per season).
Every bottling of that harvest was then skipped too.

- Drop the (year, type) unique index. The slug is now the only unique,
  public identifier: /ernten/<slug>, e.g. bluetenhonig-2026, then
  bluetenhonig-2026-07-14 for a further harvest of the same type.
- New Harvest.apicoreHarvestId as the sync key, backfilled from the old
  "apicore-<id>" slugs; the sync gives those rows a readable slug.
- Slugs are generated by one shared slugify() that normalizes Unicode
  (decomposed umlauts) and writes out umlauts. The migration repairs
  manual slugs created by the old umlaut bug ("fr-htracht-2026").
- Slug collisions are resolved by the database unique index, so parallel
  writers cannot produce duplicates; known slugs are skipped up front to
  keep the logs free of handled P2002 errors.
- Old URLs /ernten/<year>/<type> answer with 308 to the new address.

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

prisma/schema.prisma

@@ -148,9 +148,14 @@ model Address {
 model Harvest {
   id          String   @id @default(cuid())
   year        Int
-  /// z.B. "Fruehtracht", "Sommertracht"
+  /// z.B. "Fruehtracht", "Sommertracht". Bewusst NICHT eindeutig je Jahr:
+  /// dieselbe Sorte wird oft mehrmals im Jahr geerntet.
   type        String
+  /// Oeffentliche Adresse /ernten/<slug> — siehe lib/harvest-slug.ts.
   slug        String   @unique
+  /// ID der Ernte in ApiCore; Wiedererkennungs-Schluessel des Syncs
+  /// (admin/actions/apicore-sync.ts). null fuer von Hand angelegte Ernten.
+  apicoreHarvestId String? @unique
   harvestDate DateTime
   location    String   @default("Altenmuenster")
   description String
@@ -170,7 +175,8 @@ model Harvest {
   batches  Batch[]
   products Product[]
 
-  @@unique([year, type])
+  /// Nur noch fuer die Weiterleitung alter Adressen /ernten/<jahr>/<sorte>.
+  @@index([year, type])
   @@map("harvests")
 }