Add NSFW-only filtering mode for content moderation

- Add nsfw_only_filtering field to GuildSettings model
- Create database migration for new field (20260124_add_nsfw_only_filtering)
- Update AI moderation logic to respect NSFW-only mode
- Add Discord command !ai nsfwonly <true/false> for toggling mode
- Implement filtering logic in image analysis for both attachments and embeds
- Add comprehensive test suite for new functionality
- Update documentation with usage examples and feature description
- Create dedicated CI workflow for testing NSFW-only filtering feature

When enabled, only sexual/nude content is filtered while allowing:
- Violence and gore
- Harassment and bullying
- Hate speech
- Self-harm content
- Other content categories

This mode is useful for gaming communities and mature discussion
servers that have specific content policies allowing violence
but prohibiting sexual material.

src/guardden/models/guild.py

@@ -97,6 +97,7 @@ class GuildSettings(Base, TimestampMixin):
     ai_confidence_threshold: Mapped[float] = mapped_column(Float, default=0.7, nullable=False)
     ai_log_only: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     nsfw_detection_enabled: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
+    nsfw_only_filtering: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
 
     # Verification settings
     verification_enabled: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)