Implement DuplicateDetectionService pre-submit RPC orchestration

The fail-safe pattern (catch network exceptions → return non-blocking result) is critical: duplicate detection must never prevent a legitimate activity submission due to a transient network error. Document this explicitly with a code comment above the try/catch. The blocking threshold (default 0.85) should be a named parameter with a const default so it can be changed per organization in future without rebuilding the service. Keep _mapCandidate() private and test it indirectly through checkForDuplicates() — avoid making it public just to test it directly.

For the Riverpod provider, use a Provider (not StateNotifierProvider) since the service is stateless.

Write unit tests in test/data/services/duplicate_detection_service_impl_test.dart using a mock DuplicateCheckRepository (via mocktail). Test scenarios: (1) repository returns 2 candidates, one above blocking threshold → result has isBlocked=true and severity=block, (2) repository returns 1 candidate below threshold → isBlocked=false and severity=warning, (3) repository returns empty list → severity=none, (4) repository throws SocketException → service returns safe fallback result with isBlocked=false, (5) custom blocking threshold injected at 0.9 → verify threshold is applied correctly. Use bloc_test or manual async test patterns. Target 90%+ line coverage.