feat: add source file:line to WARN and ERROR log lines

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/logging.rs

@@ -0,0 +1,46 @@
+//! Custom log formatter that includes source file and line for WARN/ERROR events.
+
+use std::fmt;
+use tracing::{Event, Level, Subscriber};
+use tracing_subscriber::fmt::format::{self, FormatEvent, FormatFields};
+use tracing_subscriber::fmt::FmtContext;
+use tracing_subscriber::registry::LookupSpan;
+
+/// Event formatter that appends `[file:line]` to WARN and ERROR log lines.
+///
+/// INFO, DEBUG, and TRACE events use the default format without source location.
+pub struct ErrorLocationFormat;
+
+impl<S, N> FormatEvent<S, N> for ErrorLocationFormat
+where
+    S: Subscriber + for<'a> LookupSpan<'a>,
+    N: for<'a> FormatFields<'a> + 'static,
+{
+    fn format_event(
+        &self,
+        ctx: &FmtContext<'_, S, N>,
+        mut writer: format::Writer<'_>,
+        event: &Event<'_>,
+    ) -> fmt::Result {
+        let metadata = event.metadata();
+        let level = metadata.level();
+        let timestamp = chrono::Utc::now().format("%Y-%m-%dT%H:%M:%S%.6fZ");
+
+        // Write: timestamp + level + target
+        write!(writer, "{} {:>5} {}", timestamp, level, metadata.target())?;
+
+        // Append [file:line] for WARN and ERROR only
+        if *level <= Level::WARN {
+            if let (Some(file), Some(line)) = (metadata.file(), metadata.line()) {
+                write!(writer, " [{}:{}]", file, line)?;
+            }
+        }
+
+        write!(writer, ": ")?;
+
+        // Write the event fields (message, structured fields)
+        ctx.format_fields(writer.by_ref(), event)?;
+
+        writeln!(writer)
+    }
+}

backend/src/main.rs

@@ -4,6 +4,7 @@
 //! and starts the HTTP server. Also supports the `create-admin` CLI subcommand.
 
 mod cli;
+mod logging;
 
 use anyhow::Context;
 use clap::Parser;
@@ -27,7 +28,10 @@ async fn main() -> anyhow::Result<()> {
     // Initialize tracing
     let filter = EnvFilter::try_from_default_env()
         .unwrap_or_else(|_| EnvFilter::new("info,ai_synth_backend=debug"));
-    fmt().with_env_filter(filter).init();
+    fmt()
+        .with_env_filter(filter)
+        .event_format(logging::ErrorLocationFormat)
+        .init();
 
     let cli = Cli::parse();