fix: use valid model ID for default LLM settings (#876)

## Summary

- Default model was `"sonnet"` which isn't a valid model manifest ID
- Changed to `"claude-sonnet-4-5"` which matches the manifest
- Fixes 400 errors when creating sessions without an explicit model
(e.g. CI workflows via `ambient-action`)

## Test plan

- [ ] Create a session without specifying a model — should default to
`claude-sonnet-4-5`
- [ ] Verify PR fixer CI workflow can create sessions successfully

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Gkrumbach07

components/backend/handlers/sessions.go

@@ -608,7 +608,7 @@ func CreateSession(c *gin.Context) {
 
 	// Set defaults for LLM settings if not provided
 	llmSettings := types.LLMSettings{
-		Model:       "sonnet",
+		Model:       "claude-sonnet-4-5",
 		Temperature: 0.7,
 		MaxTokens:   4000,
 	}