fix: Add missing AWS configuration properties to env object

Fixed TypeScript build errors by properly adding AWS_REGION,
AWS_ACCESS_KEY_ID, and AWS_SECRET_ACCESS_KEY properties to
the env configuration object. These properties were being
referenced in embed.ts but were missing from cfg.ts.

Changes:
- Removed incorrect aws_model property with 3-argument str() call
- Added AWS_REGION, AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY
  as individual properties with proper default values

Fixes TypeScript errors:
- cfg.ts(53,9): Expected 2 arguments, but got 3
- embed.ts: Property 'AWS_*' does not exist errors

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: amihos

backend/src/core/cfg.ts

@@ -47,12 +47,9 @@ export const env = {
     openai_model: process.env.OM_OPENAI_MODEL,
     gemini_key:
         process.env.GEMINI_API_KEY || process.env.OM_GEMINI_API_KEY || "",
-    aws_model: str(
-        process.env.AWS_REGION,
-        process.env.AWS_ACCESS_KEY_ID,
-        process.env.AWS_SECRET_ACCESS_KEY
-    ),
-          
+    AWS_REGION: process.env.AWS_REGION || "",
+    AWS_ACCESS_KEY_ID: process.env.AWS_ACCESS_KEY_ID || "",
+    AWS_SECRET_ACCESS_KEY: process.env.AWS_SECRET_ACCESS_KEY || "",
     ollama_url: str(
         process.env.OLLAMA_URL || process.env.OM_OLLAMA_URL,
         "http://localhost:11434",