fix: resilient GDPR export sections + release-log entry

Reworks the data export per founder feedback: instead of either
silently masking failures (the original `.catch(() => [])`) or hard
failing the whole export on one bad sub-query (the prior fix), each
section now loads through a `loadSection` helper that:

  - logs the underlying error (pino + Sentry) so schema drift / query
    regressions are observable rather than invisible,
  - returns a fallback so the rest of the export still ships
    (resilient — the user isn't denied all their data over one section),
  - records the section name so the response declares itself `partial`
    with `incompleteSections`, keeping a GDPR Art. 20 export honest
    about completeness rather than passing a partial dump off as whole.

Adds the docs/release-log.md entry this PR was missing (the CI
"Enforce release log update" gate failed at ~11s because the diff
touched backend/src without a log entry).

Tests updated: a failing section now asserts 200 + partial:true +
incompleteSections, and a complete export asserts partial:false.

Author: Apexone11