feat(portal-bff): redis client foundation per ADR-0010 (#109)

## Summary

First step toward Redis-backed sessions (ADR-0010). Adds the shared `ioredis` connection that every downstream consumer (session storage, OBO token cache, …) injects via the new `REDIS_CLIENT` DI token. No session logic in this PR — that's the next one.

## What lands

- **`ioredis@^5.10.1`** as a direct dependency. Chosen by ADR-0010 for its mature Sentinel support — single-instance URL today, Sentinel-HA configuration lands with the prod infrastructure ADR.
- **[`.env.example`](apps/portal-bff/.env.example)** promotes `REDIS_URL` from its future-vars comment to an active variable, defaulting to the local Compose stack's address. The Sentinel-style keys (`REDIS_SENTINEL_HOSTS`, `REDIS_SENTINEL_NAME`, `REDIS_TLS`) stay in the future-vars comment until the prod deploy.
- **[`check-redis-config.ts`](apps/portal-bff/src/config/check-redis-config.ts)** — boot-time guard mirroring the existing four:
  - Refuses to start on missing / non-`redis(s)://` / passwordless / placeholder URLs.
  - Returns a typed `RedisConfig` with parsed `host` + `port` for downstream observability.
- **[`redis.token.ts`](apps/portal-bff/src/redis/redis.token.ts)** — `REDIS_CLIENT` string token + `Redis` type alias. Same shape as the existing `ENTRA_CONFIG` / `MSAL_CLIENT`.
- **[`redis.module.ts`](apps/portal-bff/src/redis/redis.module.ts)** — `RedisModule` factory provider:
  - Caps `maxRetriesPerRequest: 3` so an unreachable Redis surfaces a clear command-time error rather than an infinite reconnect storm.
  - Wires `connect` / `ready` / `error` / `close` / `reconnecting` events into the Pino stream under the `redis` context — easy log isolation.
  - Non-global; consumers import the module to state "I depend on Redis".
- **`main.ts`** calls `assertRedisConfig()` alongside the other three validators; **`AppModule`** imports `RedisModule`.

## Decisions worth flagging

- **`maxRetriesPerRequest: 3`** rather than the ioredis default of 20. With the default, a Redis outage masquerades as request-level timeouts spread over minutes. Capping low surfaces the outage in the first command failure — the BFF can then return 503 and recover quickly when Redis comes back.
- **Single shared client.** Pub/sub use-cases (when they appear) duplicate via `redis.duplicate()` per ioredis convention. Connect/disconnect is one socket per BFF instance.
- **No explicit shutdown hook yet.** Node's process-exit handlers and ioredis's own cleanup take care of the socket on SIGTERM / Ctrl+C. If we see stuck connections in real load, we wire `OnApplicationShutdown` + `redis.quit()`.
- **Sentinel-style config stays in the future-vars comment.** ioredis supports it natively, but plumbing it on top of the URL form complicates the validator and the factory for zero v1 payoff. Lands with the prod infrastructure ADR.

## Verification

- `nx run-many -t lint test build --projects=portal-bff` — green.
- **62 / 62 specs** (was 52; +10 — `check-redis-config` covers happy path + 6 failure modes; `redis.module` covers DI resolution against an unreachable URL plus the missing-env failure).
- Boot smoke against the local Compose stack: Pino's `redis` context shows `redis.connect` → `redis.ready` on startup; killing the Redis container produces `redis.close` / `redis.reconnecting` lines.

## What this PR explicitly does NOT do

- Mount `express-session` + `connect-redis` middleware. The next PR wires the session cookie (`__Host-portal_session`), the encrypted payload, and the lookup middleware that attaches `user` to every request.
- Plug the callback into session creation. Auth still ends with a Pino log + redirect; the SPA still sees the user anonymous on the next request.
- Sentinel / TLS configuration. Future-var keys are documented in `.env.example` for when the prod deploy lands.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #109

apps/portal-bff/src/config/check-redis-config.ts

@@ -0,0 +1,85 @@
+/**
+ * Boot-time guard for the Redis connection string. Same family as
+ * `check-database-url.ts` / `check-entra-config.ts` /
+ * `check-session-secret.ts` — fails before NestFactory if the URL
+ * is missing or obviously malformed, so the contributor sees one
+ * clear message instead of a `ioredis` reconnect storm.
+ *
+ * v1 understands the single-instance form (`redis://[user]:pass@host:port/db`).
+ * Sentinel-style configuration (`REDIS_SENTINEL_HOSTS` +
+ * `REDIS_SENTINEL_NAME` per ADR-0010) lands when the production
+ * deploy ADR ships; until then the validator only accepts URLs.
+ */
+
+const PLACEHOLDER_PASSWORD = 'redis_dev_change_me';
+
+export interface RedisConfig {
+  /** Original URL as configured; consumed by `ioredis`'s URL ctor. */
+  readonly url: string;
+  /** Parsed host (for log lines and the future readiness probe). */
+  readonly host: string;
+  /** Parsed port (defaults to 6379 when the URL omits it). */
+  readonly port: number;
+}
+
+export function assertRedisConfig(): RedisConfig {
+  const raw = process.env['REDIS_URL'];
+  if (!raw) {
+    throw new Error(
+      'REDIS_URL is not set. Copy apps/portal-bff/.env.example to ' +
+        'apps/portal-bff/.env and adjust to match infra/local/.env ' +
+        '(REDIS_PASSWORD + REDIS_PORT).',
+    );
+  }
+
+  let parsed: URL;
+  try {
+    parsed = new URL(raw);
+  } catch {
+    throw new Error(
+      `REDIS_URL is not a valid URL. Expected ` +
+        `"redis://[user]:<password>@<host>:<port>/<db>"; got: ${truncate(raw)}`,
+    );
+  }
+
+  if (parsed.protocol !== 'redis:' && parsed.protocol !== 'rediss:') {
+    throw new Error(
+      `REDIS_URL must use the "redis://" or "rediss://" scheme; got "${parsed.protocol}".`,
+    );
+  }
+
+  // The local Compose stack requires a non-default password (the
+  // compose refuses to boot without it). A REDIS_URL without one
+  // would only connect to a non-authenticated Redis — surface the
+  // mismatch up-front rather than at the first connection attempt.
+  if (!parsed.password) {
+    throw new Error(
+      `REDIS_URL has no password. The local dev stack requires a ` +
+        `password (REDIS_PASSWORD in infra/local/.env). Add it to ` +
+        `the URL's userinfo segment.`,
+    );
+  }
+
+  if (parsed.password === PLACEHOLDER_PASSWORD) {
+    throw new Error(
+      `REDIS_URL still carries the .env.example placeholder password ` +
+        `("${PLACEHOLDER_PASSWORD}"). Set a real value in ` +
+        `infra/local/.env and mirror it here.`,
+    );
+  }
+
+  const port = parsed.port ? Number(parsed.port) : 6379;
+  if (!Number.isFinite(port) || port <= 0 || port > 65535) {
+    throw new Error(`REDIS_URL has an invalid port: ${parsed.port}`);
+  }
+
+  return {
+    url: raw,
+    host: parsed.hostname,
+    port,
+  };
+}
+
+function truncate(s: string): string {
+  return s.length > 32 ? `${s.slice(0, 32)}…` : s;
+}