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

src/config/mod.rs

@@ -139,11 +139,14 @@ impl AppConfig {
             .set_default("providers.ollama.models", Vec::<String>::new())?;
 
         // Load from config file if exists
-        let config_path = config_path.unwrap_or_else(|| {
-            std::env::current_dir()
-                .unwrap_or_else(|_| PathBuf::from("."))
-                .join("config.toml")
-        });
+        // Priority: explicit path arg > LLM_PROXY__CONFIG_PATH env var > ./config.toml
+        let config_path = config_path
+            .or_else(|| std::env::var("LLM_PROXY__CONFIG_PATH").ok().map(PathBuf::from))
+            .unwrap_or_else(|| {
+                std::env::current_dir()
+                    .unwrap_or_else(|_| PathBuf::from("."))
+                    .join("config.toml")
+            });
         if config_path.exists() {
             config_builder = config_builder.add_source(File::from(config_path.clone()).format(FileFormat::Toml));
         }