feat: Custom fields — JSONB with schema per entity (PRD §5.5)

Models: CustomFieldDef (schema definitions) + CustomFieldValue (entity values)
- Per entity type: company, person, deal, project, invoice, ticket, contract
- Field types: string, number, boolean, date, select, multiselect
- Options for select/multiselect stored as JSON
- Unique constraint: (entityType, key) for definitions, (entityType, entityId, fieldDefId) for values

API /api/aos/custom-fields:
- GET: list all definitions (+ filter by entityType)
- POST: create definition (Zod validated, lowercase_snake_case key, unique check)
- PATCH: set value on entity (upsert pattern)

Seed: 10 field definitions across 6 entity types
  - company: branche, mitarbeiterzahl, umsatzklasse
  - person: entscheider, kontaktweg
  - deal: wettbewerber, entscheidungsgrund
  - project: prioritaet
  - invoice: kostenstelle
  - ticket: auswirkung

Verified: 10 defs, 3 company fields, duplicate→409, invalid key→400

prisma/schema.prisma

@@ -753,3 +753,36 @@ model Jahresarchiv {
   createdBy     String
   notes         String?
 }
+
+// ---------------------------------------------------------------------------
+// Custom Fields (PRD §5.5: "Custom fields: JSONB with a schema table per entity")
+// ---------------------------------------------------------------------------
+
+model CustomFieldDef {
+  id          String   @id @default(cuid())
+  entityType  String   // company|person|deal|project|invoice|ticket|contract
+  key         String   // field key, e.g. "branche", "lead_source_detail"
+  label       String   // display label, e.g. "Branche"
+  type        String   // string|number|boolean|date|select|multiselect
+  options     String?  // JSON array for select/multiselect
+  required    Boolean  @default(false)
+  sortOrder   Int      @default(0)
+  values     CustomFieldValue[]
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  @@unique([entityType, key])
+}
+
+model CustomFieldValue {
+  id          String   @id @default(cuid())
+  fieldDefId  String
+  fieldDef    CustomFieldDef @relation(fields: [fieldDefId], references: [id])
+  entityType  String   // denormalized for fast lookup
+  entityId    String   // the entity's ID
+  value       String?  // JSON-encoded value (string, number, bool, array)
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  @@unique([entityType, entityId, fieldDefId])
+}