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

packages/sdk/src/types.gen.ts

@@ -1544,6 +1544,7 @@ export type SettingsSettings = {
     heartbeat_enabled?: boolean;
     heartbeat_interval?: number;
     heartbeat_model_id?: string;
+    image_model_id?: string;
     language?: string;
     memory_provider_id?: string;
     reasoning_effort?: string;
@@ -1564,6 +1565,7 @@ export type SettingsUpsertRequest = {
     heartbeat_enabled?: boolean;
     heartbeat_interval?: number;
     heartbeat_model_id?: string;
+    image_model_id?: string;
     language?: string;
     memory_provider_id?: string;
     reasoning_effort?: string;