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>

nextcloud_mcp_server/document_processors/base.py

@@ -0,0 +1,117 @@
+"""Abstract base class for document processing plugins."""
+
+from abc import ABC, abstractmethod
+from typing import Any, Optional
+
+from pydantic import BaseModel
+
+
+class ProcessingResult(BaseModel):
+    """Standardized result from any document processor."""
+
+    text: str
+    """Extracted text content"""
+
+    metadata: dict[str, Any]
+    """Processor-specific metadata"""
+
+    processor: str
+    """Name of processor that handled this (e.g., 'unstructured', 'tesseract')"""
+
+    success: bool = True
+    """Whether processing succeeded"""
+
+    error: Optional[str] = None
+    """Error message if processing failed"""
+
+
+class DocumentProcessor(ABC):
+    """Abstract base class for document processing plugins.
+
+    Document processors extract text from various file formats (PDF, DOCX, images, etc.).
+    Each processor implements this interface and can be registered with the ProcessorRegistry.
+
+    Example:
+        class MyProcessor(DocumentProcessor):
+            @property
+            def name(self) -> str:
+                return "my_processor"
+
+            @property
+            def supported_mime_types(self) -> set[str]:
+                return {"application/pdf", "image/jpeg"}
+
+            async def process(self, content: bytes, content_type: str, **kwargs) -> ProcessingResult:
+                # Extract text from content
+                return ProcessingResult(text="...", metadata={}, processor=self.name)
+
+            async def health_check(self) -> bool:
+                return True
+    """
+
+    @property
+    @abstractmethod
+    def name(self) -> str:
+        """Unique identifier for this processor (e.g., 'unstructured', 'tesseract')."""
+        pass
+
+    @property
+    @abstractmethod
+    def supported_mime_types(self) -> set[str]:
+        """Set of MIME types this processor can handle.
+
+        Examples: {"application/pdf", "image/jpeg", "image/png"}
+        """
+        pass
+
+    @abstractmethod
+    async def process(
+        self,
+        content: bytes,
+        content_type: str,
+        filename: Optional[str] = None,
+        options: Optional[dict[str, Any]] = None,
+    ) -> ProcessingResult:
+        """Process a document and extract text.
+
+        Args:
+            content: Document bytes
+            content_type: MIME type of the document
+            filename: Optional filename for format detection
+            options: Processor-specific options (e.g., OCR language, strategy)
+
+        Returns:
+            ProcessingResult with extracted text and metadata
+
+        Raises:
+            ProcessorError: If processing fails
+        """
+        pass
+
+    @abstractmethod
+    async def health_check(self) -> bool:
+        """Check if processor is available and healthy.
+
+        Returns:
+            True if processor is ready to use, False otherwise
+        """
+        pass
+
+    def supports(self, content_type: str) -> bool:
+        """Check if this processor supports the given MIME type.
+
+        Args:
+            content_type: MIME type (may include parameters like "application/pdf; charset=utf-8")
+
+        Returns:
+            True if this processor can handle the type
+        """
+        # Strip parameters from content type
+        base_type = content_type.split(";")[0].strip().lower()
+        return base_type in self.supported_mime_types
+
+
+class ProcessorError(Exception):
+    """Raised when document processing fails."""
+
+    pass