refactor(log): replace stdlib log with the application JSON logger

Every log.Printf/Fatalf becomes a structured slog call through
internal/log: request handlers use the request-scoped logger
(applog.FromContext, carrying request_id), startup failures exit via a
fatal() helper that still emits JSON, and the seedsample/dumpschema CLIs
bootstrap the same JSON logger. Stale client_test expectations aligned
with the refactored wrapper-call logging (message casing, result attr,
ERROR level).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/garmin/client.go

@@ -205,15 +205,12 @@ func (c *subprocessClient) ensureStarted(ctx context.Context) error {
 	c.started = true
 	c.nextID = 0
 
-	applog.FromContext(ctx).Info("garmin wrapper spawning",
-		"python_path", pythonPath,
-	)
 	return nil
 }
 
 // roundTrip sends one request and returns its result payload, or an error
 // if the wrapper reported one. Callers must hold c.mu and have already
-// called ensureStarted. Logs exactly one "garmin wrapper call" line
+// called ensureStarted. Logs exactly one "Garmin wrapper call" line
 // regardless of outcome (see internal/applog) -- cmd/params are always
 // safe to log in full here: Garmin credentials only ever reach the
 // subprocess via env vars at spawn time (see ensureStarted), never through
@@ -228,13 +225,13 @@ func (c *subprocessClient) roundTrip(ctx context.Context, cmdName string, params
 		}
 		level := slog.LevelInfo
 		if result != nil {
-			attrs = append(attrs, slog.String("result_preview", truncate(string(result), 500)))
+			attrs = append(attrs, slog.String("result", truncate(string(result), 500)))
 		}
 		if err != nil {
-			level = slog.LevelWarn
+			level = slog.LevelError
 			attrs = append(attrs, slog.String("error", err.Error()))
 		}
-		applog.FromContext(ctx).LogAttrs(context.Background(), level, "garmin wrapper call", attrs...)
+		applog.FromContext(ctx).LogAttrs(context.Background(), level, "Garmin wrapper call", attrs...)
 	}()
 
 	c.nextID++