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

internal/db/sqlc/models.go

@@ -31,6 +31,7 @@ type Bot struct {
 	CompactionRatio     int32              `json:"compaction_ratio"`
 	CompactionModelID   pgtype.UUID        `json:"compaction_model_id"`
 	TitleModelID        pgtype.UUID        `json:"title_model_id"`
+	ImageModelID        pgtype.UUID        `json:"image_model_id"`
 	TtsModelID          pgtype.UUID        `json:"tts_model_id"`
 	BrowserContextID    pgtype.UUID        `json:"browser_context_id"`
 	Metadata            []byte             `json:"metadata"`