fix: Remove ModerationResult and unused imports from AI services

Fix ImportError caused by removed ModerationResult class.

Changes:
- Remove ModerationResult from ai/__init__.py exports
- Remove unused ContentCategory enum and parse_categories function
- Remove unused imports from ai_moderation.py
- Clean up NullProvider to only have analyze_image method

Fixes bot startup crash.

src/guardden/services/ai/factory.py

@@ -11,21 +11,11 @@ logger = logging.getLogger(__name__)
 class NullProvider(AIProvider):
     """Null provider that does nothing (for when AI is disabled)."""
 
-    async def moderate_text(self, content, context=None, sensitivity=50):
-        from guardden.services.ai.base import ModerationResult
-
-        return ModerationResult()
-
     async def analyze_image(self, image_url, sensitivity=50):
         from guardden.services.ai.base import ImageAnalysisResult
 
         return ImageAnalysisResult()
 
-    async def analyze_phishing(self, url, message_content=None):
-        from guardden.services.ai.base import PhishingAnalysisResult
-
-        return PhishingAnalysisResult()
-
     async def close(self):
         pass