fix: add SQLite busy_timeout to prevent database locked errors

Set PRAGMA busy_timeout=5000 (5 seconds) to allow SQLite to retry
when the database is locked instead of failing immediately.

This fixes race conditions when multiple goroutines try to write
simultaneously, particularly in tests where StoreObservation spawns
async cleanup goroutines.

Root cause:
- StoreObservation launches goroutine -> CleanupOldObservations
- Multiple concurrent cleanups caused "database is locked" errors
- Without busy_timeout, SQLite fails immediately on lock contention

Solution:
- Add 5-second busy timeout for automatic retry on lock
- Standard practice for concurrent SQLite usage
- Works with existing WAL mode configuration

Fixes TestObservationStore_CleanupOldObservations in CI.

internal/db/gorm/store.go

@@ -88,6 +88,11 @@ func NewStore(cfg Config) (*Store, error) {
 	if _, err := sqlDB.Exec("PRAGMA synchronous=NORMAL"); err != nil {
 		return nil, fmt.Errorf("set synchronous mode: %w", err)
 	}
+	// Set busy timeout to 5 seconds to handle concurrent writes
+	// This allows SQLite to retry when database is locked instead of failing immediately
+	if _, err := sqlDB.Exec("PRAGMA busy_timeout=5000"); err != nil {
+		return nil, fmt.Errorf("set busy timeout: %w", err)
+	}
 
 	return store, nil
 }