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>

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