feat: add NGIT_LOG_LEVEL configuration option

Add proper log level configuration following standard approach:
- CLI flag: --log-level <level>
- Environment variable: NGIT_LOG_LEVEL
- Default: info
- Supports simple levels (error, warn, info, debug, trace)
- Supports filter expressions (e.g., ngit_grasp=debug,actix_web=info)

Configuration is now consistent across all four sources:
1. src/config.rs - Config struct with log_level field
2. docs/reference/configuration.md - Full documentation
3. nix/module.nix - NixOS module with logLevel option
4. .env.example - Example configuration file

This replaces the previous RUST_LOG approach with proper integration
into the ngit-grasp configuration system, enabling trace logging from
CLI, environment variables, or NixOS configuration.

2 files changed, 13 insertions, 8 deletions

diff --git a/src/config.rs b/src/config.rs
index 271a340..df7a7ef 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -500,6 +500,10 @@ pub struct Config {
     /// Prevents connection exhaustion DoS attacks
     #[arg(long, env = "NGIT_MAX_CONNECTIONS", default_value_t = 4096)]
     pub max_connections: usize,
+
+    /// Log level for application logging
+    #[arg(long, env = "NGIT_LOG_LEVEL", default_value = "info")]
+    pub log_level: String,
 }
 
 impl Config {
@@ -782,6 +786,7 @@ impl Config {
             repository_blacklist: String::new(),
             event_blacklist: String::new(),
             max_connections: 500,
+            log_level: "debug".to_string(),
         }
     }
 }
diff --git a/src/main.rs b/src/main.rs
index 5e5b83a..105b861 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -3,8 +3,8 @@ use std::{path::PathBuf, sync::Arc};
 
 use anyhow::Result;
 use tokio::signal;
-use tracing::{error, info, warn, Level};
-use tracing_subscriber::FmtSubscriber;
+use tracing::{error, info, warn};
+use tracing_subscriber::{EnvFilter, FmtSubscriber};
 
 use ngit_grasp::{
     config::{Config, DatabaseBackend},
@@ -17,16 +17,16 @@ use ngit_grasp::{
 
 #[tokio::main]
 async fn main() -> Result<()> {
-    // Initialize tracing
+    // Load configuration first (priority: CLI flags > env vars > .env file > defaults)
+    let config = Config::load()?;
+
+    // Initialize tracing with configured log level
     let subscriber = FmtSubscriber::builder()
-        .with_max_level(Level::DEBUG)
+        .with_env_filter(EnvFilter::new(&config.log_level))
         .finish();
     tracing::subscriber::set_global_default(subscriber)?;
 
-    info!("Starting ngit-grasp with nostr-relay-builder...");
-
-    // Load configuration (priority: CLI flags > env vars > .env file > defaults)
-    let config = Config::load()?;
+    info!("Starting ngit-grasp with log level: {}", config.log_level);
 
     // Validate configuration and fail fast on fatal errors
     // Recoverable issues (e.g., malformed whitelist entries) are logged as warnings