Phase 3 Step 7: Scan Comparison Features & UX Improvements

Implemented comprehensive scan comparison functionality with historical
analysis and improved user experience for scan triggering.

Features Added:
- Scan comparison engine with ports, services, and certificates analysis
- Drift score calculation (0.0-1.0 scale) for infrastructure changes
- Side-by-side comparison UI with color-coded changes (added/removed/changed)
- Historical trend charts showing port counts over time
- "Compare with Previous" button on scan detail pages
- Scan history API endpoint for trending data

API Endpoints:
- GET /api/scans/<id1>/compare/<id2> - Compare two scans
- GET /api/stats/scan-history/<id> - Historical scan data for charts

UI Improvements:
- Replaced config file text inputs with dropdown selectors
- Added config file selection to dashboard and scans pages
- Improved delete scan confirmation with proper async handling
- Enhanced error messages with detailed validation feedback
- Added 2-second delay before redirect to ensure deletion completes

Comparison Features:
- Port changes: tracks added, removed, and unchanged ports
- Service changes: detects version updates and service modifications
- Certificate changes: monitors SSL/TLS certificate updates
- Interactive historical charts with clickable data points
- Automatic detection of previous scan for comparison

Bug Fixes:
- Fixed scan deletion UI alert appearing on successful deletion
- Prevented config file path duplication (configs/configs/...)
- Improved error handling for failed API responses
- Added proper JSON response parsing with fallback handling

Testing:
- Created comprehensive test suite for comparison functionality
- Tests cover comparison API, service methods, and drift scoring
- Added edge case tests for identical scans and missing data

web/templates/scans.html

@@ -114,13 +114,19 @@
                 <form id="trigger-scan-form">
                     <div class="mb-3">
                         <label for="config-file" class="form-label">Config File</label>
-                        <input type="text"
-                               class="form-control"
-                               id="config-file"
-                               name="config_file"
-                               placeholder="/app/configs/example.yaml"
-                               required>
-                        <div class="form-text text-muted">Path to YAML configuration file</div>
+                        <select class="form-select" id="config-file" name="config_file" required>
+                            <option value="">Select a config file...</option>
+                            {% for config in config_files %}
+                            <option value="{{ config }}">{{ config }}</option>
+                            {% endfor %}
+                        </select>
+                        <div class="form-text text-muted">
+                            {% if config_files %}
+                                Select a scan configuration file
+                            {% else %}
+                                <span class="text-warning">No config files found in /app/configs/</span>
+                            {% endif %}
+                        </div>
                     </div>
                     <div id="trigger-error" class="alert alert-danger" style="display: none;"></div>
                 </form>