🐛 fix: emit all config-loading logs in correct JSON format from the start (#16)

## Summary

Closes #15

When `logging.json: true` (or `DLC_LOGGING_JSON=true`), the logger was unconditionally initialised to console/text format at the top of `LoadConfig()`, so early log lines — most visibly **"Config file loaded"** — were always written as human-readable text regardless of configuration.

## Root cause

Classic chicken-and-egg: the format flag lives inside the config that is being loaded. The format-switch block only ran *after* `v.Unmarshal()`, too late for the config-file log.

## Changes

### `pkg/config/config.go`
- Add `peekJSONLogging()`: resolves the JSON flag **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. This mirrors Viper's own priority order without parsing the full config twice.
- Apply the resolved format immediately and emit **"Logging configured"** as the very first log line.
- Remove the now-redundant format-switch block that ran after `Unmarshal()`.

### `scripts/start-server.sh`, `test-graceful-shutdown.sh`, `test-opentelemetry.sh`
- Replace hardcoded `PROJECT_DIR` path with a dynamic `SCRIPTS_DIR=$(dirname $(realpath ${BASH_SOURCE[0]}))` derivation so scripts work from any worktree or clone location.

## Test plan
- [x] `go test ./pkg/...` — all pass
- [x] `scripts/test-graceful-shutdown.sh` — all JSON valid, all startup logs present
- [x] Manual smoke test: first line is `{"level":"info",...,"message":"Logging configured"}`, every line is valid JSON

Reviewed-on: #16
Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

cmd/server/main.go

@@ -48,8 +48,10 @@ func main() {
 		log.Fatal().Err(err).Msg("Failed to load configuration")
 	}
 
-	// Create readiness context to control readiness state
-	readyCtx, readyCancel := context.WithCancel(context.Background())
+	// Create readiness context to control readiness state.
+	// CancelableContext exposes Cancel() so that Server.Run() can cancel
+	// readiness at the start of graceful shutdown (before the propagation sleep).
+	readyCtx, readyCancel := server.NewCancelableContext(context.Background())
 	defer readyCancel()
 
 	// Create and run server
@@ -57,4 +59,5 @@ func main() {
 	if err := server.Run(); err != nil {
 		log.Fatal().Err(err).Msg("Server failed")
 	}
+	log.Trace().Msg("Server exited")
 }