feat(helm): Add document chunking configuration

Add support for configurable document chunking parameters to Helm chart
to match docker-compose and application capabilities.

Changes:
1. values.yaml:
   - Add documentChunking section with chunkSize (512) and chunkOverlap (50)
   - Include comprehensive comments explaining chunking strategies
   - Positioned between vectorSync and qdrant sections

2. templates/deployment.yaml:
   - Add DOCUMENT_CHUNK_SIZE and DOCUMENT_CHUNK_OVERLAP env vars
   - Always set (not conditional), used by vector sync processor
   - Environment variables follow same pattern as config.py defaults

3. README.md:
   - Add documentChunking parameter table in Vector Search section
   - Document chunking strategies (small/medium/large chunks)
   - Explain overlap recommendations (10-20% of chunk size)

Validation:
- helm lint: Passes
- helm template: Environment variables correctly generated
- Custom values: Work as expected (tested with chunkSize=1024)
- Always present: Not conditional on vectorSync.enabled

This maintains feature parity between Helm and docker-compose deployments,
allowing users to tune chunking for their embedding models and use cases.

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

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

charts/nextcloud-mcp-server/templates/deployment.yaml

@@ -158,6 +158,11 @@ spec:
             - name: VECTOR_SYNC_QUEUE_MAX_SIZE
               value: {{ .Values.vectorSync.queueMaxSize | quote }}
             {{- end }}
+            # Document Chunking (always set, used by vector sync processor)
+            - name: DOCUMENT_CHUNK_SIZE
+              value: {{ .Values.documentChunking.chunkSize | quote }}
+            - name: DOCUMENT_CHUNK_OVERLAP
+              value: {{ .Values.documentChunking.chunkOverlap | quote }}
             # Qdrant Vector Database
             {{- if eq .Values.qdrant.mode "network" }}
             # Network mode: Use dedicated Qdrant service