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/engine/src/self-healing.ts

@@ -1626,6 +1626,22 @@ export class SelfHealingManager {
             log.log(`Maintenance batch 1 step "cleanup-old-mail" succeeded — messagesDeleted=${messagesDeleted}`);
           },
         },
+        {
+          name: "prune-operational-logs",
+          fn: async () => {
+            const days = Number(settings.operationalLogRetentionDays ?? 0);
+            if (!Number.isFinite(days) || days <= 0) {
+              log.log("Maintenance batch 1 step \"prune-operational-logs\" skipped — operationalLogRetentionDays is not enabled");
+              return;
+            }
+            const { deletedTotal, deletedByTable } = this.store.pruneOperationalLogs(days * 86_400_000);
+            const detail = Object.entries(deletedByTable)
+              .filter(([, n]) => n > 0)
+              .map(([t, n]) => `${t}=${n}`)
+              .join(" ");
+            log.log(`Maintenance batch 1 step "prune-operational-logs" succeeded — deleted=${deletedTotal}${detail ? ` (${detail})` : ""}`);
+          },
+        },
         { name: "checkpoint-wal", fn: () => Promise.resolve(this.checkpointWal()) },
         { name: "enforce-worktree-cap", fn: () => this.enforceWorktreeCap() },
       ];