The hardcoded `static final String IMPORT_DIR = "/import"` was the only
non-`@Value` configurable input in MassImportService — every column
index next to it is wired through `app.import.col.*`. Lifts the
contract from infrastructure (compose bind mount) into application
config (`app.import.dir`), with `/import` as the default so the existing
bind-mount path keeps working.
Addresses review feedback from Markus and Felix on #526.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>