perf(backend): batch digest and push writes, read in parallel, share parsers, delete dead code - #2444

🟢 Confidence 10/10 · safe to merge. Batched writes and the org purge are pinned by new PGlite tests; the three concurrency edits only reorder independent reads. (The kickoff's "changed since then" list named files this diff does not con…
_{no findings · tests covered · risk high}

Batches the daily membership sweep and the device push stamp into multi-row writes, runs three groups of independent reads concurrently, and purges fix-verification rows with the org. The new writes and the purge are exercised by tests; no correctness, scoping or data-loss defect found.

• reconcileSubscriptions upserts 500 members at a time, deduped by (org, user)
• markPushed stamps every device id in one IN (...) UPDATE
• ErrorIssueVerifications moves into the org-delete purge
• getBearerToken and parseIngestKeyEncryptionKey collapse to one copy each

Production impact

_{0d1d10a · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}