Add concurrent scan operation tests (18 tests, all passing)

✅ COMPLETE: 18/18 tests passing

Test Coverage:
- Concurrent scan prevention: Second scan blocked, multiple attempts handled, lock released after completion
- Scan cancellation: Cancel active scan, cancel when none active, cancelled scans in history, new scan after cancellation
- Database race conditions: AsyncIO lock prevents races, scan state consistency with concurrent reads, thread-safe history updates
- Scan state consistency: is_scanning flag consistency, current_scan object consistency, status API consistency, concurrent status checks
- Scheduler prevention: Scheduler skips rescan if already running, properly sets scan_in_progress flag
- AnimeService: Ignores concurrent rescan requests

All concurrent operation scenarios validated with proper lock management and state consistency.

docs/instructions.md

@@ -542,12 +542,21 @@ All TIER 2 high priority core UX features have been completed:
 
 #### Edge Case Tests
 
-- [ ] **Create tests/unit/test_concurrent_scans.py** - Concurrent scan operation tests
-    - Test multiple simultaneous scan requests handled gracefully
-    - Test scan cancellation/interruption handling
-    - Test database race condition prevention during scans
-    - Test scan state consistency with concurrent requests
-    - Target: 100% of concurrent operation scenarios covered
+- [x] **Create tests/unit/test_concurrent_scans.py** - Concurrent scan operation tests ✅ COMPLETE
+    - Note: 18/18 tests passing - comprehensive concurrent scan handling coverage
+    - Coverage: Concurrent scan prevention (5 tests), scan cancellation (4 tests), database race conditions (3 tests), scan state consistency (4 tests), scheduler prevention (2 tests)
+    - Test ✅ Second scan blocked while first running
+    - Test ✅ Multiple scan attempts properly handled
+    - Test ✅ Scan lock released after completion/error
+    - Test ✅ Cancel active scan
+    - Test ✅ New scan after cancellation
+    - Test ✅ AsyncIO lock prevents race conditions
+    - Test ✅ Scan state consistency with concurrent reads
+    - Test ✅ Thread-safe history updates
+    - Test ✅ is_scanning flag consistency
+    - Test ✅ Scheduler skips if scan already running
+    - Test ✅ AnimeService ignores concurrent requests
+    - Target achieved: ✅ COMPLETE - all concurrent operation scenarios covered
 
 - [ ] **Create tests/unit/test_download_retry.py** - Download retry logic tests
     - Test automatic retry after download failure