docs(deploy): Caddy front-end example + walkthrough (forgejo-mcp-broker-r2c)

Adds deploy/caddy/Caddyfile and docs/deploy-caddy.md, the front-end
half of the production deployment that pairs with deploy-podman.md.

Caddyfile:
  - reverse_proxy with flush_interval -1 (mandatory for /mcp SSE)
  - structured JSON access log to a separate file
  - validated with `caddy validate` and formatted with `caddy fmt`
  - omits explicit X-Forwarded-{For,Proto,Host} since Caddy forwards
    them by default (caddy validate flags them as redundant)

deploy-caddy.md walks operators through:
  - why a reverse proxy at all (TLS, SSE, future rate limits)
  - the host-header trap and why FJMCP_BROKER_PUBLIC_URL is the
    trusted source of issuer URLs (cross-references the existing
    TestDiscovery_IssuerIgnoresHostHeader regression)
  - SSE buffering as the most common deployment foot-gun
  - optional rate-limit recipe via caddy-ratelimit (defers to backlog
    issue -ttl)
  - troubleshooting for the four failure modes the broker has actually
    seen during dev: wrong issuer, buffered SSE, unreachable upstream,
    TLS conflict

README updated to link both deploy guides and the deploy/ subtree.

Closes forgejo-mcp-broker-r2c.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/deploy-podman.md

@@ -85,7 +85,7 @@ curl -fsS http://127.0.0.1:8080/healthz | jq .
 ```
 
 A healthy response includes `{"status":"ok","store":"ok",…}`. Wire
-Caddy in front next; see `docs/deploy-caddy.md`.
+Caddy in front next; see [`deploy-caddy.md`](deploy-caddy.md).
 
 ## 5. Persist state across image upgrades