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

spec/docs.go

@@ -13029,6 +13029,9 @@ const docTemplate = `{
                 "heartbeat_model_id": {
                     "type": "string"
                 },
+                "image_model_id": {
+                    "type": "string"
+                },
                 "language": {
                     "type": "string"
                 },
@@ -13085,6 +13088,9 @@ const docTemplate = `{
                 "heartbeat_model_id": {
                     "type": "string"
                 },
+                "image_model_id": {
+                    "type": "string"
+                },
                 "language": {
                     "type": "string"
                 },