Major security and code quality improvements

Security:
- Add input validation for all API endpoints (frequency, lat/lon, device, gain, ppm)
- Add HTML escaping utility to prevent XSS attacks
- Add path traversal protection for log file configuration
- Add proper HTTP status codes for error responses (400, 409, 503)

Performance:
- Reduce SSE keepalive overhead (30s interval instead of 1s)
- Add centralized SSE stream utility with optimized keepalive
- Add DataStore class for thread-safe data with automatic cleanup

New Features:
- Add data export endpoints (/export/aircraft, /export/wifi, /export/bluetooth)
- Support for both JSON and CSV export formats
- Add process cleanup on application exit (atexit handlers)
- Label Iridium module as demo mode with clear warnings

Code Quality:
- Create utils/validation.py for centralized input validation
- Create utils/sse.py for SSE stream utilities
- Create utils/cleanup.py for memory management
- Add safe_terminate() and register_process() for process management
- Improve error handling with proper logging throughout routes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

utils/__init__.py

@@ -1,6 +1,15 @@
 # Utility modules for INTERCEPT
 from .dependencies import check_tool, check_all_dependencies, TOOL_DEPENDENCIES
-from .process import cleanup_stale_processes, is_valid_mac, is_valid_channel, detect_devices
+from .process import (
+    cleanup_stale_processes,
+    is_valid_mac,
+    is_valid_channel,
+    detect_devices,
+    safe_terminate,
+    register_process,
+    unregister_process,
+    cleanup_all_processes,
+)
 from .logging import (
     get_logger,
     app_logger,
@@ -12,3 +21,22 @@ from .logging import (
     satellite_logger,
     iridium_logger,
 )
+from .validation import (
+    escape_html,
+    validate_latitude,
+    validate_longitude,
+    validate_frequency,
+    validate_device_index,
+    validate_gain,
+    validate_ppm,
+    validate_hours,
+    validate_elevation,
+    validate_wifi_channel,
+    validate_mac_address,
+    validate_positive_int,
+    sanitize_callsign,
+    sanitize_ssid,
+    sanitize_device_name,
+)
+from .sse import sse_stream, format_sse, clear_queue
+from .cleanup import DataStore, CleanupManager, cleanup_manager, cleanup_dict