Add dashboard statistics module and refactor views for metrics handling

- Introduced a new `stats.py` module to encapsulate dashboard statistics building and cache key constants.
- Refactored `views.py` to utilize the new `build_stats` function for constructing metrics context, improving code organization and readability.
- Updated Prometheus query handling to streamline metrics fetching with a new `fetch_dashboard_metrics` function.
- Enhanced test cases to reflect changes in metrics fetching and context building, ensuring accurate functionality.
- Added new HTML templates for displaying detailed resource allocation and flavor statistics on the dashboard.

templates/dashboard/_chart_migrations.html

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+<!-- ANALYSIS VISUALIZATION -->
+<section aria-label="CPU distribution chart">
+    <div class="card bg-base-100 shadow-sm">
+        <div class="card-body p-5">
+            <h3 class="text-lg font-semibold mb-4">CPU Distribution (Current vs Projected)</h3>
+            <div class="h-48">
+                <canvas id="cpuDistributionChart"></canvas>
+            </div>
+            <div class="flex items-center justify-center gap-3 mt-3">
+                <div class="flex items-center gap-1 text-xs">
+                    <div class="w-3 h-0.5 bg-success"></div>
+                    <span class="text-success">Mean: <span id="currentCpuMean">0</span>%</span>
+                </div>
+                <div class="flex items-center gap-1 text-xs">
+                    <div class="w-3 h-0.5 bg-error/60"></div>
+                    <span class="text-error/60">±0.5σ: <span id="currentCpuStd">0</span>%</span>
+                </div>
+            </div>
+        </div>
+    </div>
+</section>
+
+<!-- MIGRATION ACTIONS -->
+<section aria-label="Migration actions">
+<div class="card bg-base-100 shadow-sm">
+    <div class="card-body p-6">
+        <div class="flex items-center justify-between mb-4">
+            <h3 class="text-lg font-semibold">Migration Actions</h3>
+            <div class="badge badge-neutral badge-sm" id="migrationCount">Select audit</div>
+        </div>
+
+        <div class="overflow-x-auto">
+            <table class="table table-zebra table-sm">
+                <thead>
+                    <tr class="bg-base-200">
+                        <th class="text-xs font-medium">Instance</th>
+                        <th class="text-xs font-medium">Source → Destination</th>
+                        <th class="text-xs font-medium">Flavor</th>
+                        <th class="text-xs font-medium">Impact</th>
+                    </tr>
+                </thead>
+                <tbody id="migrationTableBody" class="text-sm">
+                    <tr>
+                        <td colspan="4" class="text-center py-6 text-base-content/60">
+                            No audit selected. Load an audit to view migration recommendations.
+                        </td>
+                    </tr>
+                </tbody>
+            </table>
+        </div>
+    </div>
+</div>
+</section>