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/deployment.yaml

@@ -88,8 +88,8 @@ spec:
             - name: NEXTCLOUD_PUBLIC_ISSUER_URL
               value: {{ include "nextcloud-mcp-server.publicIssuerUrl" . | quote }}
             {{- if .Values.auth.multiUserBasic.enableOfflineAccess }}
-            # Background operations with app passwords
-            - name: ENABLE_OFFLINE_ACCESS
+            # Background operations with app passwords (replaces deprecated ENABLE_OFFLINE_ACCESS)
+            - name: ENABLE_BACKGROUND_OPERATIONS
               value: "true"
             - name: TOKEN_STORAGE_DB
               value: {{ .Values.auth.multiUserBasic.tokenStorageDb | quote }}
@@ -182,16 +182,16 @@ spec:
               value: {{ .Values.documentProcessing.custom.types | quote }}
             {{- end }}
             {{- end }}
-            # Vector Sync
-            - name: VECTOR_SYNC_ENABLED
-              value: {{ .Values.vectorSync.enabled | quote }}
-            {{- if .Values.vectorSync.enabled }}
+            # Semantic Search (replaces deprecated VECTOR_SYNC_ENABLED)
+            - name: ENABLE_SEMANTIC_SEARCH
+              value: {{ .Values.semanticSearch.enabled | quote }}
+            {{- if .Values.semanticSearch.enabled }}
             - name: VECTOR_SYNC_SCAN_INTERVAL
-              value: {{ .Values.vectorSync.scanInterval | quote }}
+              value: {{ .Values.semanticSearch.scanInterval | quote }}
             - name: VECTOR_SYNC_PROCESSOR_WORKERS
-              value: {{ .Values.vectorSync.processorWorkers | quote }}
+              value: {{ .Values.semanticSearch.processorWorkers | quote }}
             - name: VECTOR_SYNC_QUEUE_MAX_SIZE
-              value: {{ .Values.vectorSync.queueMaxSize | quote }}
+              value: {{ .Values.semanticSearch.queueMaxSize | quote }}
             {{- end }}
             # Document Chunking (always set, used by vector sync processor)
             - name: DOCUMENT_CHUNK_SIZE