feat(helm): migrate to new environment variable naming convention

Replace deprecated environment variables with new consolidated names:
- VECTOR_SYNC_ENABLED → ENABLE_SEMANTIC_SEARCH
- ENABLE_OFFLINE_ACCESS → ENABLE_BACKGROUND_OPERATIONS

Update values.yaml structure:
- Rename 'vectorSync' section to 'semanticSearch'
- Update descriptions to emphasize BM25 hybrid search

Benefits:
- Aligns with application-level config consolidation
- Clearer naming: "semantic search" vs "vector sync"
- Maintains backward compatibility via application deprecation handling
- Automatic enablement of background ops when semantic search enabled in multi-user modes

Updated files:
- values.yaml: Renamed vectorSync → semanticSearch
- deployment.yaml: New env var names with deprecation comments
- NOTES.txt: Updated deployment notes
- README.md: Updated documentation and examples

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

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

charts/nextcloud-mcp-server/templates/NOTES.txt

@@ -69,12 +69,12 @@ Your Nextcloud MCP Server has been deployed in {{ .Values.auth.mode }} authentic
    {{- end }}
 {{- end }}
 
-{{- if .Values.vectorSync.enabled }}
+{{- if .Values.semanticSearch.enabled }}
 
-5. Vector Search & Semantic Capabilities:
-   - Vector Sync: Enabled
-   - Scan Interval: {{ .Values.vectorSync.scanInterval }}s
-   - Processor Workers: {{ .Values.vectorSync.processorWorkers }}
+5. Semantic Search & Vector Capabilities:
+   - Semantic Search: Enabled
+   - Scan Interval: {{ .Values.semanticSearch.scanInterval }}s
+   - Processor Workers: {{ .Values.semanticSearch.processorWorkers }}
    {{- if .Values.qdrant.enabled }}
    - Qdrant: Deployed as subchart ({{ .Release.Name }}-qdrant:6333)
    {{- else }}