feat(api): make Redis a required cache dependency

Redis becomes mandatory infrastructure (PostgreSQL-parity): the
REDIS_ENABLED flag, NullCache backend, and every conditional caching
code path are removed. /health/ready reports not_ready (503) when
Redis is unreachable and cached endpoints return 503 instead of
silently degrading; invalidation helpers never raise after a
committed write.

The bundled redis container joins the core profile only via
docker-compose.dev.yml (production points REDIS_* at an external
instance, same no-shadowing pattern as postgres), the backend suite
runs against real Redis (test-redis on 127.0.0.1:55433, CI redis:8
service container, per-xdist-worker key prefixes), and operator docs
describe the required-dependency setup (upgrading.md gains the
v0.20.0 section).

docker-compose.test-db.yml

@@ -1,5 +1,5 @@
 # ==========================================================================
-# Throwaway PostgreSQL instance for the backend test suite (pytest).
+# Throwaway PostgreSQL + Redis instances for the backend test suite (pytest).
 #
 # NOT part of the application stack. Start/stop with:
 #   make test-db-up
@@ -11,7 +11,8 @@
 # The meshcorehub role owns the meshcorehub_test database (image entrypoint),
 # which is exactly the privilege the per-xdist-worker test schemas need
 # (CREATE SCHEMA) — no superuser and no CREATEDB required. The data directory
-# is a tmpfs mount, so nothing survives `down`.
+# is a tmpfs mount, so nothing survives `down`. Redis (required cache
+# infrastructure) is published on 55433 with the same tmpfs treatment.
 # ==========================================================================
 name: meshcore-hub-test-db
 
@@ -36,3 +37,16 @@ services:
       interval: 5s
       timeout: 5s
       retries: 10
+
+  test-redis:
+    image: redis:8-alpine
+    container_name: meshcore-hub-test-redis
+    ports:
+      - "127.0.0.1:55433:6379"
+    tmpfs:
+      - /data
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 5s
+      timeout: 5s
+      retries: 10