fix: add null guards for getChromaSync() when Chroma is disabled (#1336)

When CLAUDE_MEM_CHROMA_ENABLED=false, getChromaSync() returns null.
Two call sites were missing null guards, causing "null is not an object"
errors on every UserPromptSubmit / session init.

Fixes #1294

Vibe-coded by Ousama Ben Younes

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/services/worker/http/routes/SessionRoutes.ts

@@ -356,7 +356,7 @@ export class SessionRoutes extends BaseRouteHandler {
       // Sync user prompt to Chroma
       const chromaStart = Date.now();
       const promptText = latestPrompt.prompt_text;
-      this.dbManager.getChromaSync().syncUserPrompt(
+      this.dbManager.getChromaSync()?.syncUserPrompt(
         latestPrompt.id,
         latestPrompt.memory_session_id,
         latestPrompt.project,