feat: anonymous pre-market release per unit

Add anonymous flag to schattenmarktRelease on PropertyUnit and Property.
When a unit is enabled in the pre-market panel, an Anonym toggle appears
inline — hides address and property name in the matching feed, showing
only area, type and region to prospective tenants.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/domain/unit.ts

@@ -33,6 +33,7 @@ export interface PropertyUnit {
   schattenmarktRelease?: {
     enabled: boolean
     availableFrom?: string  // ISO date
+    anonymous?: boolean     // hide address/name in pre-market feed
   }
 }