feat: add unified provider architecture with Amazon Bedrock support

Refactored LLM provider infrastructure to support sustainable additions of new providers with both embedding and text generation capabilities.

## Major Changes

### Unified Provider Architecture (ADR-015)
- Created `nextcloud_mcp_server/providers/` with unified Provider ABC
- Providers now support optional capabilities (embeddings and/or generation)
- Auto-detection registry with priority: Bedrock → Ollama → Simple
- Backward compatible - existing code continues to work

### New Providers
- **BedrockProvider**: Full Amazon Bedrock integration
  - Embeddings: Titan Embed, Cohere Embed models
  - Generation: Claude, Llama, Titan Text, Mistral models
  - Model-specific request/response handling
  - AWS credential chain integration
- **OllamaProvider**: Migrated with both capabilities support
- **AnthropicProvider**: Moved from test code to production providers
- **SimpleProvider**: Migrated in-memory fallback provider

### Breaking Changes
None - full backward compatibility maintained:
- `embedding.get_embedding_service()` still works
- RAG evaluation tests updated to use unified providers
- All existing tests pass (127 unit tests)

### Testing
- Added 9 comprehensive Bedrock unit tests with mocked boto3
- All existing unit tests pass
- Type checking (ty) and linting (ruff) pass
- Verified backward compatibility

### Documentation
- `docs/ADR-015-unified-provider-architecture.md`: Comprehensive ADR
- `docs/bedrock-setup.md`: AWS setup guide with IAM permissions
- `CLAUDE.md`: Updated with provider architecture section

### Dependencies
- Added `boto3>=1.35.0` to dev dependencies (optional)

## Environment Variables

### Bedrock
- `AWS_REGION`: AWS region (e.g., "us-east-1")
- `BEDROCK_EMBEDDING_MODEL`: Model ID for embeddings
- `BEDROCK_GENERATION_MODEL`: Model ID for generation
- `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY`: Optional credentials

### Ollama
- `OLLAMA_BASE_URL`: API URL
- `OLLAMA_EMBEDDING_MODEL`: Embedding model (default: "nomic-embed-text")
- `OLLAMA_GENERATION_MODEL`: Generation model

## AWS Bedrock Permissions Required

Minimal IAM policy:
```json
{
  "Effect": "Allow",
  "Action": ["bedrock:InvokeModel"],
  "Resource": ["arn:aws:bedrock:*::foundation-model/*"]
}
```

See `docs/bedrock-setup.md` for detailed setup instructions.

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

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

nextcloud_mcp_server/embedding/service.py

@@ -1,57 +1,30 @@
-"""Embedding service with provider detection."""
+"""Embedding service with provider detection.
+
+DEPRECATED: This module is maintained for backward compatibility.
+New code should use nextcloud_mcp_server.providers.get_provider() directly.
+"""
 
 import logging
-import os
 
-from .base import EmbeddingProvider
+from nextcloud_mcp_server.providers import get_provider
+
 from .bm25_provider import BM25SparseEmbeddingProvider
-from .ollama_provider import OllamaEmbeddingProvider
-from .simple_provider import SimpleEmbeddingProvider
 
 logger = logging.getLogger(__name__)
 
 
 class EmbeddingService:
-    """Unified embedding service with automatic provider detection."""
+    """
+    Unified embedding service with automatic provider detection.
+
+    DEPRECATED: This class wraps the new unified provider infrastructure
+    for backward compatibility. New code should use
+    nextcloud_mcp_server.providers.get_provider() directly.
+    """
 
     def __init__(self):
         """Initialize embedding service with auto-detected provider."""
-        self.provider = self._detect_provider()
-
-    def _detect_provider(self) -> EmbeddingProvider:
-        """
-        Auto-detect available embedding provider.
-
-        Checks environment variables in order:
-        1. OLLAMA_BASE_URL - Use Ollama provider (production)
-        2. OPENAI_API_KEY - Use OpenAI provider (future)
-        3. Fallback to SimpleEmbeddingProvider (testing/development)
-
-        Returns:
-            Configured embedding provider
-        """
-        # Ollama provider (production)
-        ollama_url = os.getenv("OLLAMA_BASE_URL")
-        if ollama_url:
-            logger.info(f"Using Ollama embedding provider: {ollama_url}")
-            return OllamaEmbeddingProvider(
-                base_url=ollama_url,
-                model=os.getenv("OLLAMA_EMBEDDING_MODEL", "nomic-embed-text"),
-                verify_ssl=os.getenv("OLLAMA_VERIFY_SSL", "true").lower() == "true",
-            )
-
-        # OpenAI provider (future implementation)
-        # openai_key = os.getenv("OPENAI_API_KEY")
-        # if openai_key:
-        #     return OpenAIEmbeddingProvider(api_key=openai_key)
-
-        # Fallback to simple provider for development/testing
-        logger.warning(
-            "No embedding provider configured (OLLAMA_BASE_URL or OPENAI_API_KEY not set). "
-            "Using SimpleEmbeddingProvider for testing/development. "
-            "For production, configure an external embedding service."
-        )
-        return SimpleEmbeddingProvider(dimension=384)
+        self.provider = get_provider()
 
     async def embed(self, text: str) -> list[float]:
         """