Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/config/config.go

@@ -70,11 +70,11 @@ func LoadFromViper(conf *JobSearchConfig) {
 		}
 		value, ok := viperValue(key, keyType)
 		if !ok {
-			slog.Error(fmt.Sprintf("unknown key type: %s", keyType))
+			slog.Error("unknown config key type", "key", key, "kind", keyType.String())
 			continue
 		}
 		if err := conf.SetValue(key, value); err != nil {
-			slog.Warn("ignoring invalid config value", "key", key, "err", err)
+			slog.Warn("ignoring invalid config value", "key", key, "error", err)
 		}
 	}
 }