perf(inbox-queue): parallel batch processing to drain queue backlog

Sequential await in batch loop meant 20 items took 20x per-item time.
Switch to Promise.allSettled so all BATCH_SIZE workers run concurrently —
each claims its own item atomically via findOneAndUpdate, no double-processing.
Also bumps BATCH_SIZE from 10→20. Incoming rate was exceeding drain rate,
causing unbounded queue growth.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/inbox-queue.js

@@ -76,7 +76,7 @@ export async function enqueueActivity(collections, { activityType, actorUrl, obj
   });
 }
 
-const BATCH_SIZE = 10;
+const BATCH_SIZE = 20;
 const POLL_INTERVAL_MS = 1_000;
 
 /**
@@ -91,15 +91,18 @@ export function startInboxProcessor(collections, getCtx, handle) {
     try {
       const ctx = getCtx();
       if (!ctx) return;
-      for (let i = 0; i < BATCH_SIZE; i++) {
-        const hadWork = await processNextItem(collections, ctx, handle);
-        if (!hadWork) break; // Queue empty, stop early
-      }
+      // Run BATCH_SIZE workers in parallel — each atomically claims its own item
+      // via findOneAndUpdate, so there is no double-processing risk.
+      await Promise.allSettled(
+        Array.from({ length: BATCH_SIZE }, () =>
+          processNextItem(collections, ctx, handle),
+        ),
+      );
     } catch (error) {
       console.error("[inbox-queue] Processor error:", error.message);
     }
   }, POLL_INTERVAL_MS);
 
-  console.info(`[ActivityPub] Inbox queue processor started (${POLL_INTERVAL_MS}ms interval, batch size ${BATCH_SIZE})`);
+  console.info(`[ActivityPub] Inbox queue processor started (${POLL_INTERVAL_MS}ms interval, batch size ${BATCH_SIZE}, parallel)`);
   return intervalId;
 }