feat: add Qdrant local mode support with in-memory and persistent storage

Adds flexible Qdrant deployment modes to reduce infrastructure requirements
for local development and smaller deployments:

**Configuration Changes:**
- Add QDRANT_LOCATION environment variable (mutually exclusive with QDRANT_URL)
- Three modes: network (URL), in-memory (:memory:, default), persistent (file path)
- Settings dataclass validation via __post_init__ ensures mutual exclusivity
- API key warning when set in local mode (ignored, only for network mode)

**Client Initialization:**
- Auto-detect mode: network (url + api_key) vs local (:memory: or path=)
- In-memory: AsyncQdrantClient(":memory:") - zero config default
- Persistent: AsyncQdrantClient(path="/app/data/qdrant") - file storage
- Network: AsyncQdrantClient(url, api_key) - production mode

**Docker Compose Updates:**
- Qdrant service moved to optional profile (--profile qdrant)
- MCP service uses QDRANT_LOCATION=:memory: by default
- Added mcp-data volume for persistent storage (/app/data)
- No hard dependency on qdrant service

**Documentation:**
- Comprehensive configuration guide in docs/configuration.md
- All three modes documented with pros/cons
- Docker Compose examples for each mode
- Environment variable reference table

**Tests:**
- 13 new config validation tests (mutual exclusivity, defaults, warnings)
- Persistent mode integration test (create, close, reopen, verify persistence)
- All 82 unit tests + 5 smoke tests pass

**Breaking Change:**
- Default changed from QDRANT_URL=http://qdrant:6333 to QDRANT_LOCATION=:memory:
- Simplifies local development (no external service needed)
- Production deployments: explicitly set QDRANT_URL or QDRANT_LOCATION

Related: ADR-007 background vector sync implementation

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

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

docker-compose.yml

@@ -74,10 +74,10 @@ services:
     depends_on:
       app:
         condition: service_healthy
-      qdrant:
-        condition: service_healthy
     ports:
       - 127.0.0.1:8000:8000
+    volumes:
+      - mcp-data:/app/data
     environment:
       - NEXTCLOUD_HOST=http://app:80
       - NEXTCLOUD_USERNAME=admin
@@ -88,9 +88,13 @@ services:
       - VECTOR_SYNC_SCAN_INTERVAL=10
       - VECTOR_SYNC_PROCESSOR_WORKERS=1
 
-      # Qdrant configuration
-      - QDRANT_URL=http://qdrant:6333
-      - QDRANT_API_KEY=${QDRANT_API_KEY:-my_secret_api_key}
+      # Qdrant configuration (three modes):
+      # 1. Network mode: Set QDRANT_URL=http://qdrant:6333 (requires qdrant service)
+      # 2. In-memory mode: Set QDRANT_LOCATION=:memory: (default if nothing set)
+      # 3. Persistent local: Set QDRANT_LOCATION=/app/data/qdrant (stored in mcp-data volume)
+      - QDRANT_LOCATION=:memory:
+      # - QDRANT_URL=http://qdrant:6333  # Uncomment for network mode
+      # - QDRANT_API_KEY=${QDRANT_API_KEY:-my_secret_api_key}  # Only for network mode
       - QDRANT_COLLECTION=nextcloud_content
 
       # Ollama configuration (optional - uses SimpleEmbeddingProvider if not set)
@@ -215,6 +219,8 @@ services:
       interval: 10s
       timeout: 5s
       retries: 10
+    profiles:
+      - qdrant
 
 volumes:
   nextcloud:
@@ -224,3 +230,4 @@ volumes:
   keycloak-tokens:
   keycloak-oauth-storage:
   qdrant-data:
+  mcp-data: