fix(config): wire up LLM_PROXY__CONFIG_PATH env var and fix database path in service

The app never read the LLM_PROXY__CONFIG_PATH env var, so the systemd
service couldn't find /etc/llm-proxy/config.toml and fell back to
./data/llm_proxy.db (owned by root, readonly for llmproxy user).

- Add LLM_PROXY__CONFIG_PATH support to config loader (checks env var
  before falling back to ./config.toml)
- Add LLM_PROXY__DATABASE__PATH to service env so the DB path always
  resolves to /var/lib/llm-proxy/llm_proxy.db regardless of config

deploy.sh

@@ -264,6 +264,7 @@ WorkingDirectory=$INSTALL_DIR
 EnvironmentFile=$ENV_FILE
 Environment="RUST_LOG=info"
 Environment="LLM_PROXY__CONFIG_PATH=$CONFIG_DIR/config.toml"
+Environment="LLM_PROXY__DATABASE__PATH=$DATA_DIR/llm_proxy.db"
 ExecStart=$INSTALL_DIR/target/release/$APP_NAME
 Restart=on-failure
 RestartSec=5