docs: add issue file for re-enabling caddy http3

Refs: #31

Author: josecelano

docs/issues/ISSUE-31-reenable-caddy-http3-and-document-rationale.md

@@ -0,0 +1,93 @@
+# Re-enable Caddy HTTP/3 and Document ISSUE-29 Rationale
+
+**Issue**: [#31](https://github.com/torrust/torrust-tracker-demo/issues/31)
+**Related**:
+[#29](https://github.com/torrust/torrust-tracker-demo/issues/29),
+[#30](https://github.com/torrust/torrust-tracker-demo/issues/30),
+[ISSUE-29-research-high-cpu-load-after-udp-fix.md](ISSUE-29-research-high-cpu-load-after-udp-fix.md)
+
+## Overview
+
+ISSUE-29 removed Caddy UDP port mapping `443:443/udp` (HTTP/3 over QUIC) during a controlled
+production experiment. The observations showed no measurable CPU improvement after disabling
+HTTP/3, while service availability remained stable.
+
+This follow-up issue proposes re-enabling HTTP/3 at the Caddy edge and documenting why this
+reversal is intentional. The goal is to restore HTTP/3 capability for present and future clients,
+while keeping a controlled rollback path if resource cost or reliability regresses.
+
+This issue also clarifies the protocol boundary in the current architecture:
+
+- Edge protocol (client -> Caddy) can include HTTP/3.
+- Backend protocol (Caddy -> tracker/grafana) remains reverse-proxy HTTP and does not require
+  tracker native HTTP/3 support.
+
+## Problem Statement
+
+The current ISSUE-29 wording can be read as "HTTP/3 disabled for hygiene" even though the
+experiment outcome was only that disabling HTTP/3 did not fix CPU pressure.
+
+Keeping HTTP/3 disabled by default may also block automatic support for clients that prefer
+or require HTTP/3 in the future. Since this demo already runs a Caddy edge proxy, re-enabling
+UDP 443 is a low-complexity way to restore HTTP/3 capability without changing backend services.
+
+The change should therefore be treated as a product-capability decision with operational
+guardrails, not as a CPU-remediation tactic.
+
+## Goals
+
+1. Re-enable Caddy UDP 443 publish mapping for HTTP/3 at the edge.
+2. Keep backend application topology unchanged.
+3. Record explicitly that ISSUE-29 did not show CPU benefit from disabling HTTP/3.
+4. Document why re-enable is being done now (future compatibility/capability) and how rollback
+   will be handled if needed.
+
+## Proposed Change
+
+1. Re-add `"443:443/udp"` in Caddy service ports in
+   `server/opt/torrust/docker-compose.yml`.
+2. Apply the same change on live `/opt/torrust/docker-compose.yml` and recreate only Caddy.
+3. Observe immediate, T+1h, and T+next-day checkpoints with the same metrics used in ISSUE-29.
+
+## Rollback Triggers
+
+If any trigger is met after re-enable, revert by removing `"443:443/udp"` again and record
+the rollback in evidence:
+
+1. Caddy CPU increases by more than 20% sustained for 24h vs pre-change baseline.
+2. Host load average increases by more than 15% sustained for 24h vs pre-change baseline.
+3. New external availability regression appears on tracked HTTP1 or UDP1 endpoints.
+
+## Deliverables
+
+- Compose change that re-enables Caddy UDP 443 publish mapping.
+- Evidence notes for post-change observations (immediate, T+1h, T+next-day).
+- Updated ISSUE-29 wording that clearly separates:
+  - measured performance result,
+  - capability/product decision,
+  - rollback criteria and operational safeguards.
+
+## Implementation Plan
+
+- [ ] Re-add `"443:443/udp"` for Caddy in `server/opt/torrust/docker-compose.yml`.
+- [ ] Apply only that change on the live server and recreate only Caddy.
+- [ ] Validate Caddy health and confirm host UDP 443 listener exists after deploy.
+- [ ] Capture immediate post-change metrics: `mpstat`, `docker stats`, Prometheus HTTP1/UDP1
+      rates, and `newtrackon.com/raw` sample.
+- [ ] Capture T+1h and T+next-day checkpoints with the same metrics.
+- [ ] Evaluate rollback triggers; if triggered, revert and record evidence.
+- [ ] Update ISSUE-29 text to explain why the earlier disablement is being reversed now.
+- [ ] Ensure ISSUE-29 states backend services do not need native HTTP/3 for edge HTTP/3 support.
+- [ ] Run `./scripts/lint.sh` and fix any markdown/cspell issues.
+
+## Acceptance Criteria
+
+- [ ] Caddy HTTP/3 edge capability is re-enabled via `443:443/udp` mapping.
+- [ ] Immediate, T+1h, and T+next-day evidence snapshots are recorded.
+- [ ] No rollback trigger is met during the observation window, or rollback is executed and
+      documented if a trigger is met.
+- [ ] ISSUE-29 explicitly states that disabling HTTP/3 did not reduce CPU in prior observations.
+- [ ] ISSUE-29 explicitly states why HTTP/3 was re-enabled and under which conditions it may be
+      disabled again.
+- [ ] Documentation clearly states edge HTTP/3 is independent from backend native HTTP/3 support.
+- [ ] All changed files pass `./scripts/lint.sh`.