feat: add image generation model and generate_image agent tool

Bots can now be configured with an image generation model (must have
image-output compatibility). When set, the agent exposes a generate_image
tool that calls the model via Twilight AI SDK, saves the result to the
bot container filesystem, and returns the file path.

- Add image_model_id column to bots table (migration 0053)
- Update settings SQL queries, service, and types
- New ImageGenProvider tool provider in internal/agent/tools/
- Wire provider in both cmd/agent and cmd/memoh entry points
- Add image model selector to frontend bot settings with compat filtering
- Regenerate swagger, SDK types, and sqlc code

db/migrations/0053_add_image_model.down.sql

@@ -0,0 +1,3 @@
+-- 0053_add_image_model (rollback)
+-- Remove image_model_id column from bots table
+ALTER TABLE bots DROP COLUMN IF EXISTS image_model_id;