Reads first 4 bytes of each candidate file before upload; rejects any
file whose header does not match %PDF (0x25 0x50 0x44 0x46). Skipped
files are counted and collected in ImportStatus.skippedFiles so
operators can see what was rejected without querying Loki.
Breaking: ImportStatus record gains skipped + skippedFiles fields.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>