Harden fusion.db against recurring corruption

Root cause: node:sqlite SIGSEGVs inside pager_write leave the B-tree
malformed in a way that still opens but fails integrity checks; large
operational-log tables widen the write window where the crash strikes.

- backup: verify every copy with PRAGMA quick_check, quarantine corrupt
  copies as *.corrupt, and never rotate out the last verified-good backup
- db: add Database.recoverIfCorrupt() startup guard (wired into
  TaskStore.init, disk-backed only, opt out via FUSION_DISABLE_DB_AUTORECOVER)
  that rebuilds a malformed db via sqlite3 .recover, preserving the corrupt
  original; also fixes the latent `.recover main` invalid-option bug that made
  recoverDatabase() always fail
- db: drop lost_and_found* scratch tables on init; add pruneOperationalLogs()
- settings: add operationalLogRetentionDays (default 30, 0 = off) and prune
  activityLog/agentLogEntries/runAuditEvents/agentHeartbeats during maintenance
- dashboard: expose retention in Settings -> Backups -> Database Maintenance

Tests: backup 59/59, db 135/135 (incl. real corrupt->recover->reopen),
self-healing cleanup/corruption 10/10, settings 77/77, SettingsModal 460/460.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/SettingsModal.tsx

@@ -6106,6 +6106,31 @@ export function SettingsModal({
               )}
             </div>
 
+            <h4 className="settings-section-heading settings-section-heading--spaced">Database Maintenance</h4>
+            <div className="form-group">
+              <label htmlFor="operationalLogRetentionDays">Operational log retention</label>
+              <select
+                id="operationalLogRetentionDays"
+                className="select"
+                value={form.operationalLogRetentionDays ?? 0}
+                onChange={(e) =>
+                  setForm((f) => ({ ...f, operationalLogRetentionDays: Number(e.target.value) || 0 }))
+                }
+              >
+                <option value={0}>Off</option>
+                <option value={30}>30 days</option>
+                <option value={60}>60 days</option>
+                <option value={90}>90 days</option>
+                <option value={180}>180 days</option>
+                <option value={365}>365 days</option>
+              </select>
+              <small>
+                Prune append-only operational logs (activity log, agent logs, run audit, heartbeats) older than this
+                many days during periodic maintenance. Keeps the database from growing without bound — large databases
+                are slower to checkpoint and more prone to corruption. Default: 30 days.
+              </small>
+            </div>
+
             <h4 className="settings-section-heading">Memory Backups</h4>
             <div className="form-group">
               <label htmlFor="memoryBackupEnabled" className="checkbox-label">