feat(infra): local serve-static profile — Caddy reverse proxy for the prod build (#97)

## Summary

Add a Caddy reverse proxy behind a new `--profile serve-static` so a contributor can exercise the production build locally with the per-locale routing the on-prem reverse proxy will use (per ADR-0019). Closes the gap surfaced by PR #96: the locale switcher / accessibility route fusion / cookie plumbing all need a prod-faithful local setup, and `nx serve-static` falls short (no SPA fallback per locale, no smart `/` redirect, exposes the `http-server` directory-listing footgun we hit during the perf-gate fix in PR #92).

## What lands

- **[`infra/local/Caddyfile`](infra/local/Caddyfile)** — explicit `route` block:
  - `GET /` → 302 to `/{locale}/` based on `Accept-Language`, falling back to `/fr/` (APF audience).
  - `/fr/*` → `dist/.../browser/fr/` with SPA fallback to `fr/index.html`.
  - `/en/*` → mirror.
  - Catch-all → 302 to `/fr/`.
- **[`infra/local/dev.compose.yml`](infra/local/dev.compose.yml)** — new `serve-static` service on the `serve-static` profile. Bind-mounts the Caddyfile and `dist/apps/portal-shell/browser/` read-only. Port 4200, overridable via `SERVE_STATIC_PORT`.
- **[`infra/local/.env.example`](infra/local/.env.example)** — adds `SERVE_STATIC_PORT=4200`.
- **[`infra/local/dev.sh`](infra/local/dev.sh)** — registers `serve-static` in `ALL_PROFILES` so `dev.sh down|status|logs` catches the new container, and `dev.sh up serve-static` works.
- **[`infra/README.md`](infra/README.md)** — file row, workflow snippet, cheat-sheet row, and a service-endpoint row with the `nx build … -c=production` prerequisite called out.

## Workflow

```
pnpm exec nx build portal-shell --configuration=production
./infra/local/dev.sh up serve-static
open http://localhost:4200/   # → /fr/ or /en/ per Accept-Language
```

## Decision worth flagging

Used **Caddy** rather than nginx or Traefik. Reason: minimal Caddyfile, single binary, no daemon config drift, sensible defaults (TLS off explicitly for local-only). Same family of choice as the rest of `infra/local/` — small, single-purpose images.

`redir` in a Caddyfile is **ambiguous** when the first arg starts with `/`: Caddy reads it as a path matcher rather than a redirect target. Using `redir * /fr/ 302` (explicit `*` matcher) avoids the gotcha. Documented inline in the Caddyfile via a comment block.

## What this PR explicitly does NOT do

- Wire TLS. Local convenience only, binds to `localhost`.
- Replace `nx run portal-shell:serve-static` (still used by Lighthouse CI in `ci:perf`).
- Set the `__Host-portal_locale` cookie or honour it for the smart redirect. Cookie handling needs the BFF route (ADR-0019 future PR).
- Land an on-prem reverse-proxy ADR. The on-prem infra ADR is phase 3b.

## Verified locally

| Probe | Expected | Actual |
|---|---|---|
| `GET / -H 'Accept-Language: fr'` | 302 `/fr/` | ✓ |
| `GET / -H 'Accept-Language: en'` | 302 `/en/` | ✓ |
| `GET /unknown` | 302 `/fr/` | ✓ |
| `GET /fr/deep/route` | 200 (SPA fallback to `fr/index.html`) | ✓ |
| `GET /fr/favicons/favicon.svg` | 200 (asset under locale folder) | ✓ |
| `/fr/index.html` markup | `lang="fr"`, `<base href="/fr/">` | ✓ |
| `/en/index.html` markup | `lang="en"`, `<base href="/en/">` | ✓ |

## Test plan

- [x] `docker compose -f infra/local/dev.compose.yml --profile serve-static config` validates clean.
- [x] `dev.sh up serve-static` brings the container up; `dev.sh status` lists it; `dev.sh stop serve-static` brings it down.
- [x] Routing probes above all pass.
- [ ] Manual: build + serve-static + click the locale switcher → URL becomes `/{other-locale}/`, the matching bundle boots, no console errors. (Verifies PR #95 + #96 end-to-end against a prod-faithful proxy.)
- [ ] Manual: `/fr/accessibilite` → router-level redirect to `/fr/accessibility` (verifies PR #94 under SPA fallback).
- [ ] Manual: `Accept-Language: en` in browser settings → root URL lands on `/en/`.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #97

infra/local/Caddyfile

@@ -0,0 +1,74 @@
+# Local manual-test reverse proxy for `portal-shell`'s production
+# build, simulating what the on-prem reverse proxy will do (per
+# ADR-0019 §"URL strategy"):
+#
+#   - GET /            → smart redirect to /{locale}/ based on
+#                        Accept-Language (cookie support lands when
+#                        the BFF route is in place). Falls back to
+#                        `/fr/` since the APF audience is francophone.
+#   - GET /fr/<any>    → serve `dist/.../browser/fr/<any>`, with SPA
+#                        fallback to `fr/index.html` for deep links.
+#   - GET /en/<any>    → mirror for the EN bundle.
+#   - everything else  → redirect to `/fr/` (avoids the http-server
+#                        directory-listing footgun we hit in the perf
+#                        gate while the previous fix landed).
+#
+# Used by:
+#   - `./infra/local/dev.sh up serve-static` (after building the
+#     production bundle with `nx build portal-shell -c=production`).
+#   - Convenient for testing the locale switcher end-to-end, the
+#     accessibility-route redirect, dark-mode preference plumbing,
+#     etc. — anything that needs both locale folders served at their
+#     own prefix.
+
+{
+	auto_https off
+	admin off
+}
+
+:4200 {
+	encode gzip zstd
+
+	log {
+		output stdout
+		format console
+		level INFO
+	}
+
+	# Group the locale-routing logic in an explicit `route` block so
+	# directive ordering follows source order, not Caddy's default
+	# directive priority — handle / handle_path / redir would
+	# otherwise interleave in surprising ways.
+	route {
+		# Smart redirect for the bare root.
+		@root path /
+		handle @root {
+			@prefers_en header Accept-Language en*
+			redir @prefers_en /en/ 302
+			redir * /fr/ 302
+		}
+
+		# French bundle. `handle_path` strips `/fr` before passing the
+		# request through, so `try_files` looks at `<path>` relative
+		# to the bundle root (`/srv/dist/fr/`). Missing files fall
+		# back to `index.html`, exactly like a production deep-link
+		# (e.g. /fr/accessibility) would do via a real proxy.
+		handle_path /fr/* {
+			root * /srv/dist/fr
+			try_files {path} /index.html
+			file_server
+		}
+
+		# English bundle, mirror of the above.
+		handle_path /en/* {
+			root * /srv/dist/en
+			try_files {path} /index.html
+			file_server
+		}
+
+		# Anything that didn't match a locale prefix falls back to FR
+		# home — same defaulting as the bare root above. Keeps
+		# typo'd URLs from landing on a 404.
+		redir * /fr/ 302
+	}
+}