Fix Issue 10: Document error handling pattern

- Analyzed error handling - found complete exception hierarchy already exists
- Confirmed global exception handlers registered and working
- Documented dual error handling pattern in ARCHITECTURE.md section 4.5:
  * HTTPException for simple validation and HTTP-level errors
  * Custom AniWorldAPIException for business logic with rich context
- Clarified when to use each type with examples
- Finding: Error handling was already well-structured, just needed documentation

Architecture Decision: Dual pattern is intentional and correct.
Tests passing (auth flow verified)

docs/ARCHITECTURE.md

@@ -468,6 +468,75 @@ def get_download_service() -> DownloadService:
     return _download_service_instance
 ```
 
+### 4.5 Error Handling Pattern
+
+**Architecture Decision**: Dual error handling approach based on exception source.
+
+The application uses two complementary error handling mechanisms:
+
+1. **FastAPI HTTPException** - For simple validation and HTTP-level errors
+2. **Custom Exception Hierarchy** - For business logic and service-level errors with rich context
+
+#### Exception Hierarchy
+
+```python
+# Base exception with HTTP status mapping
+AniWorldAPIException(message, status_code, error_code, details)
+├── AuthenticationError (401)
+├── AuthorizationError (403)  
+├── ValidationError (422)
+├── NotFoundError (404)
+├── ConflictError (409)
+├── BadRequestError (400)
+├── RateLimitError (429)
+└── ServerError (500)
+    ├── DownloadError
+    ├── ConfigurationError
+    ├── ProviderError
+    └── DatabaseError
+```
+
+#### When to Use Each
+
+**Use HTTPException for:**
+- Simple parameter validation (missing fields, wrong type)
+- Direct HTTP-level errors (401, 403, 404 without business context)
+- Quick endpoint-specific failures
+
+**Use Custom Exceptions for:**
+- Service-layer business logic errors (AnimeServiceError, ConfigServiceError)
+- Errors needing rich context (details dict, error codes)
+- Errors that should be logged with specific categorization
+- Cross-cutting concerns (authentication, authorization, rate limiting)
+
+**Example:**
+
+```python
+# Simple validation - Use HTTPException
+if not series_key:
+    raise HTTPException(status_code=400, detail="series_key required")
+
+# Business logic error - Use custom exception
+try:
+    await anime_service.add_series(series_key)
+except AnimeServiceError as e:
+    raise ServerError(
+        message=f"Failed to add series: {e}",
+        error_code="ANIME_ADD_FAILED",
+        details={"series_key": series_key}
+    )
+```
+
+#### Global Exception Handlers
+
+All custom exceptions are automatically handled by global middleware that:
+- Converts exceptions to structured JSON responses
+- Logs errors with appropriate severity
+- Includes request ID for tracking
+- Provides consistent error format
+
+**Source**: [src/server/exceptions/\_\_init\_\_.py](../src/server/exceptions/__init__.py), [src/server/middleware/error_handler.py](../src/server/middleware/error_handler.py)
+
 Source: [src/server/services/download_service.py](../src/server/services/download_service.py)
 
 ---