fix(persistence,rest): pre-size bulk-submit byte progress via HEAD

[angela-helios]

Closes #874

Stacked on #849 (which now carries #851's merge); merge that one first.

What

On a multi-file manifest the byte-progress denominator was learned lazily — each file's size arrived only when its stream opened — so the percentage kept recomputing against a partial total and every newly opened file yanked the bar backwards. Observed live on the #448 Postgres leg's 20-file run: 99% → 54% → 95% → 34% → ….

SubmitInputFetcher gains file_size() — a best-effort HEAD returning the advertised Content-Length, with a default implementation returning None so every existing mock keeps compiling — and the worker sums all output files up front, storing the complete denominator before ingestion starts. Fallbacks stay honest:

• any file with an unknown size → lazy accumulation exactly as before;
• encrypted (JWE) submissions skip pre-sizing entirely (wire length ≠ decrypted length);
• HEAD requests deliberately skip Accept-Encoding: gzip so the size matches the decompressed bytes the ingest counts.

New worker test: a two-file manifest lands bytes_total == bytes_processed == sum of both files through the pre-sized path. Suites: worker units (6), rest bulk_submit (22) + bulk_submit_jwe (6).

[angela-helios]

Follow-up pushed (0f716d4): live testing on the #448 MongoDB leg caught a bug in the first commit. reqwest's Response::content_length() is the body size hint — a HEAD response has no payload, so it reported 0 for every file regardless of the Content-Length header. The presize summed to zero and, worse, the presized flag then suppressed the lazy per-file fallback: byte progress silently vanished on every backend (bytes_processed counted fine, bytes_total stuck at 0). The size now comes from the Content-Length header itself, and a zero-sum presize no longer claims the denominator. Re-verified live: single-file manifest on SQLite lands bytes_total == bytes_processed == file size, and the multi-file MongoDB run reports a monotone percentage.