Add scheduled read-only reconciliation previews and suppression drift reports

Problem/Motivation

The optional reconciliation module supports reviewed imports, but operators must initiate comparisons manually. Missed webhook history can persist unnoticed between reviews.

Proposed resolution

Build an opt-in scheduler around the existing read-only provider reader that produces reviewable drift reports. Reuse current import safeguards; do not automatically remove local suppression or change provider lists.

Acceptance criteria

• Use per-source locks, bounded pagination, resumable cursors and configurable schedules with no overlapping duplicate runs.
• Handle rate limits, timeouts, expired credentials and partial pages; distinguish incomplete scans from an empty provider result.
• Summarize new provider suppression evidence and local/provider differences without inferring consent from absence.
• Require existing review/apply authorization for local imports and preserve complaint/manual/unsubscribe protections.
• Expire report data under an explicit retention policy and redact logs; test scheduling, restart, drift and concurrent webhook arrival.

Scope and dependencies

Optional follow-up after stable release; not a blocker for #3621224: Complete the 1.0.0 stable release verification gates. Builds on #3621136: Offer optional read-only Postmark reconciliation for missed webhook history; the completed original issue remains fixed.