perf: bound relay background concurrency to 3

Replaces unbounded `Promise.all(relays.map(...))` calls with `pMap` using a concurrency limit of 3 for background operations like history hydration, metadata fetching, and DM sending.
This prevents network saturation and potential main-thread jank when many relays are configured.
Includes benchmark verification script `benchmarks/relay_concurrency_repro.mjs`.
Audited upload docs and confirmed accuracy.

Co-authored-by: PR0M3TH3AN <[email protected]>

TEST_LOG.md

@@ -0,0 +1,28 @@
+# Test Log
+
+## Environment
+- Branch: unstable
+- Node version: v22.22.0
+
+## Manual Verification
+
+### Benchmark: Relay Concurrency
+- Script: `benchmarks/relay_concurrency_repro.mjs`
+- Before: Unbounded (20 requests)
+- After: Bounded (3 requests)
+- Result: PASS
+
+### Unit Tests
+- Command: `npm run test:unit`
+- Result: PASS (13 tests passed in app-batch-fetch-profiles, etc., 12 tests passed in NostrClient)
+- Note: `nostr-tools` warning in benchmark is expected in Node env.
+
+## Docs Audit
+- Page: `content/docs/guides/upload-content.md`
+- Claims Verified:
+  - File types: Matches HTML accept attribute.
+  - Size limits: 2GB recommendation matches browser RAM constraints.
+  - Upload methods: Direct, External, Magnet supported.
+  - CORS: Automatic configuration verified in `js/services/s3Service.js`.
+  - HTTPS: Enforced in `js/services/videoNotePayload.js` and docs.
+- Result: No discrepancies found.