When stdout is connected to a TTY, use tracing_subscriber's pretty
format with colors instead of single-line JSON. Non-TTY (container
logs, pipes) continue to output JSON for log aggregation.
Override auto-detection via APP_LOG_FORMAT=json|pretty.
Also adds APP_LOG_PRETTY=true to use serde_json::to_string_pretty
for human-readable JSON output (useful for development/debugging).
Add MsgJsonFormat custom event formatter that outputs JSON with _msg as
the first field, required by VictoriaLogs for full-text search. HTTP
middleware stores interpolated "METHOD /path" in thread-local buffer
for the formatter to read on span-close events.
