fix: add TARGET_API_KEY, LLM_BACKEND, LLM_API_BASE_PATH to .env schema (Light-Heart-Labs#595)

All three are written to .env by Phase 06 and present in .env.example
but missing from .env.schema.json. Schema validation crashes the
installer.

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

Author: Lightheartdevs

dream-server/.env.schema.json

@@ -31,6 +31,20 @@
       "description": "URL where all services send LLM requests",
       "default": "http://llama-server:8080"
     },
+    "LLM_BACKEND": {
+      "type": "string",
+      "description": "Inference backend: llama-server or lemonade",
+      "default": "llama-server"
+    },
+    "LLM_API_BASE_PATH": {
+      "type": "string",
+      "description": "Base API path for the inference backend",
+      "default": "/v1"
+    },
+    "TARGET_API_KEY": {
+      "type": "string",
+      "description": "API key for Privacy Shield upstream target (set to LITELLM_KEY in lemonade mode)"
+    },
     "ANTHROPIC_API_KEY": {
       "type": "string",
       "description": "Anthropic API key (cloud/hybrid modes)"