fix(graph-analysis): handle NaN in quality scoring for graphs with no edges

CHANGES:
- backend/services/local-graph-analysis.cjs:
  - Add isNaN() checks for modularity (lines 307, 313-315)
  - Default NaN values to 0 to prevent quality score from being NaN
- backend/tests/results/.gitkeep:
  - Create results directory for test outputs (fixes CI failures)

BEFORE:
- Graphs with isolated nodes (no edges) produced NaN quality scores
- drive-webhook.test.cjs and google-workspace-validation.test.cjs failed on CI due to missing results directory

AFTER:
- Quality scores default to 0 when graph has no edges
- structure-lens-v2.test.cjs passes (8/8 tests)
- Results directory exists for test outputs

VERIFICATION:
node backend/tests/structure-lens-v2.test.cjs → 8 passed, 0 failed

IMPACT: Fixes test failures in PR #52 and #53

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

Author: Michael

backend/services/local-graph-analysis.cjs

@@ -304,11 +304,16 @@ function calculateQualityScore(graph, centrality, communityDetails, gaps) {
   const metrics = {
     nodeCount: graph.order,
     edgeCount: graph.size,
-    modularity: communityDetails.modularity,
+    modularity: isNaN(communityDetails.modularity) ? 0 : communityDetails.modularity,
     gapCount: gaps.length,
     avgCentrality: Object.values(centrality).reduce((a, b) => a + b, 0) / graph.order
   };
 
+  // Handle NaN avgCentrality (when graph has no edges)
+  if (isNaN(metrics.avgCentrality)) {
+    metrics.avgCentrality = 0;
+  }
+
   // Higher modularity = better separated topics (50 points)
   const modularityScore = Math.min(metrics.modularity * 50, 50);