refactor: Transform document parsing into pluggable processor architecture

Refactors PR #190's hardcoded Unstructured.io integration into a flexible,
extensible plugin system supporting multiple text extraction engines.

- **`DocumentProcessor` ABC**: Abstract interface for all processors
- **`ProcessorRegistry`**: Central registry for discovery and routing
- **`ProcessingResult`**: Standardized output format across processors

- **`UnstructuredProcessor`**: Refactored from `UnstructuredClient`
- **`TesseractProcessor`**: Local OCR for images (lightweight alternative)
- **`CustomHTTPProcessor`**: Generic wrapper for custom HTTP APIs

- New `get_document_processor_config()` returns structured config
- Supports enabling/disabling individual processors
- Per-processor configuration via environment variables
- **Breaking Change**: `ENABLE_UNSTRUCTURED_PARSING` replaced with:
  - `ENABLE_DOCUMENT_PROCESSING=true/false` (master switch)
  - `ENABLE_UNSTRUCTURED=true/false` (per-processor)
  - `ENABLE_TESSERACT=true/false`
  - `ENABLE_CUSTOM_PROCESSOR=true/false`

- `parse_document()` now uses `ProcessorRegistry`
- Auto-selects appropriate processor based on MIME type
- Processor priority system (Unstructured=10, Tesseract=5, Custom=1)

- `initialize_document_processors()` registers processors at startup
- Integrated into both BasicAuth and OAuth lifespans
- Graceful degradation if processors fail to initialize

```env
ENABLE_DOCUMENT_PROCESSING=false

ENABLE_UNSTRUCTURED=false
UNSTRUCTURED_API_URL=http://unstructured:8000
UNSTRUCTURED_STRATEGY=auto  # auto|fast|hi_res
UNSTRUCTURED_LANGUAGES=eng,deu

ENABLE_TESSERACT=false
TESSERACT_LANG=eng

ENABLE_CUSTOM_PROCESSOR=false
CUSTOM_PROCESSOR_URL=http://localhost:9000/process
CUSTOM_PROCESSOR_TYPES=application/pdf,image/jpeg
```

- **Removed**: `tests/test_unstructured_config.py` (legacy tests)
- **Added**: `tests/unit/test_document_processor_config.py`
  - 7 unit tests for new config system
  - Tests individual and multi-processor configurations

- **Added**:
  - `nextcloud_mcp_server/document_processors/__init__.py`
  - `nextcloud_mcp_server/document_processors/base.py`
  - `nextcloud_mcp_server/document_processors/registry.py`
  - `nextcloud_mcp_server/document_processors/unstructured.py`
  - `nextcloud_mcp_server/document_processors/tesseract.py`
  - `nextcloud_mcp_server/document_processors/custom_http.py`
  - `tests/unit/test_document_processor_config.py`

- **Modified**:
  - `nextcloud_mcp_server/config.py` - New plugin config system
  - `nextcloud_mcp_server/app.py` - Processor initialization
  - `nextcloud_mcp_server/utils/document_parser.py` - Uses registry
  - `nextcloud_mcp_server/server/webdav.py` - Import updates
  - `env.sample` - New configuration format
  - `docker-compose.yml` - (profile changes from previous work)

- **Removed**:
  - `nextcloud_mcp_server/client/unstructured_client.py` - Replaced by UnstructuredProcessor
  - `tests/test_unstructured_config.py` - Replaced with new tests

✅ **Extensible**: Add processors without modifying core code
✅ **Testable**: Mock processors for unit tests
✅ **Configurable**: Enable only needed processors
✅ **Flexible**: Choose fast (Tesseract) vs accurate (Unstructured)
✅ **Opt-in**: Disabled by default, no mandatory dependencies

Users upgrading from PR #190 need to update environment variables:
```bash
ENABLE_UNSTRUCTURED_PARSING=true

ENABLE_DOCUMENT_PROCESSING=true
ENABLE_UNSTRUCTURED=true
```

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

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

docker-compose.yml

@@ -58,6 +58,8 @@ services:
       - 127.0.0.1:8002:8000
     # Unstructured API runs on port 8000 internally
     # We expose it on 8002 externally to avoid conflict
+    profiles:
+      - text-extraction
 
   mcp:
     build: .
@@ -65,19 +67,12 @@ services:
     restart: always
     depends_on:
       - app
-      - unstructured
     ports:
       - 127.0.0.1:8000:8000
     environment:
       - NEXTCLOUD_HOST=http://app:80
       - NEXTCLOUD_USERNAME=admin
       - NEXTCLOUD_PASSWORD=admin
-      - ENABLE_UNSTRUCTURED_PARSING=true
-      - UNSTRUCTURED_API_URL=http://unstructured:8000
-      - UNSTRUCTURED_STRATEGY=hi_res
-      - UNSTRUCTURED_LANGUAGES=deu,eng
-    #volumes:
-      #- ./nextcloud_mcp_server:/app/nextcloud_mcp_server:ro
 
   mcp-oauth:
     build: .