🐛 fix: emit all config-loading logs in correct format from the start

The logger was initialised to console format unconditionally, so the
"Config file loaded" message (and similar early logs) were always written
as human-readable text even when JSON logging was configured.

Root cause: classic chicken-and-egg — the format flag lives inside the
config that is being loaded.

Fix: add peekJSONLogging() which resolves the format before any log is
emitted by (1) checking DLC_LOGGING_JSON directly via os.Getenv, then
(2) doing a minimal throwaway Viper pre-read of the config file for the
logging.json key.  The redundant format-switch block that ran after
Unmarshal() is removed.

Also add the "Logging configured" log as the very first line, and
replace the hardcoded PROJECT_DIR path in start-server.sh,
test-graceful-shutdown.sh, and test-opentelemetry.sh with a dynamic
derivation from BASH_SOURCE[0].

Closes #15

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

scripts/test-graceful-shutdown.sh

@@ -7,7 +7,8 @@
 set -e
 
 # Configuration
-PROJECT_DIR="/Users/gabrielradureau/Work/Vibe/dance-lessons-coach"
+SCRIPTS_DIR=$(dirname "$(realpath "${BASH_SOURCE[0]}")")
+PROJECT_DIR=$(dirname "$SCRIPTS_DIR")
 SERVER_CMD="./scripts/start-server.sh"
 LOG_FILE="server.log"
 PID_FILE="server.pid"