New tests/fapp_flow.rs with 3 integration tests:
- full_fapp_flow_announce_query_reserve_confirm: Complete flow
from therapist announcement through patient reservation to
confirmation with E2E encryption
- fapp_rejection_flow: Tests the rejection case
- fapp_query_filters: Tests Fachrichtung, PLZ, and other filters
FappRouter additions:
- register_therapist_key(): public method for key registration
- store_announce(): public method for storing announcements
Total tests: 217 (198 lib + 3 fapp_flow + 16 multi_node)
