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/conversations.sql.go

@@ -511,7 +511,7 @@ WITH updated AS (
   SET display_name = $1,
       updated_at = now()
   WHERE bots.id = $2
-  RETURNING id, owner_user_id, display_name, avatar_url, timezone, is_active, status, language, reasoning_enabled, reasoning_effort, chat_model_id, search_provider_id, memory_provider_id, heartbeat_enabled, heartbeat_interval, heartbeat_prompt, heartbeat_model_id, compaction_enabled, compaction_threshold, compaction_ratio, compaction_model_id, title_model_id, tts_model_id, browser_context_id, metadata, created_at, updated_at, acl_default_effect
+  RETURNING id, owner_user_id, display_name, avatar_url, timezone, is_active, status, language, reasoning_enabled, reasoning_effort, chat_model_id, search_provider_id, memory_provider_id, heartbeat_enabled, heartbeat_interval, heartbeat_prompt, heartbeat_model_id, compaction_enabled, compaction_threshold, compaction_ratio, compaction_model_id, title_model_id, image_model_id, tts_model_id, browser_context_id, metadata, created_at, updated_at, acl_default_effect
 )
 SELECT
   updated.id AS id,