feat(stalwart): use Valkey for in-memory store, improve health probes

- Replace PostgreSQL in-memory store with Valkey (Redis-compatible) for
  better performance on rate limiting, distributed locks, and OAuth codes
- Add single-replica Valkey deployment with no persistence (data is transient)
- Switch liveness/readiness probes to HTTP GET /healthz/live and
  /healthz/ready on port 8080 per official Kubernetes probe documentation
- Update webadmin resource URL to use artifactapi proxy instead of direct
  GitHub download

apps/base/stalwart/resources/config.toml

@@ -67,7 +67,7 @@ permissive-cors = false
 [webadmin]
 path = "/var/lib/stalwart/webadmin"
 auto-update = true
-resource = "https://github.com/stalwartlabs/webadmin/releases/latest/download/webadmin.zip"
+resource = "https://artifactapi.k8s.syd1.au.unkin.net/generic/github/stalwartlabs/webadmin/releases/latest/download/webadmin.zip"
 
 # PostgreSQL store (via CNPG pooler)
 [store."postgresql"]
@@ -104,6 +104,11 @@ compression = "lz4"
 [store."s3".purge]
 frequency = "30 5 *"
 
+# Valkey in-memory store (rate limiting, locks, OAuth codes, greylisting)
+[store."valkey"]
+type = "redis"
+urls = ["redis://stalwart-valkey.stalwart.svc.cluster.local:6379"]
+
 # Storage assignment
 [storage]
 data = "postgresql"
@@ -111,7 +116,7 @@ fts = "postgresql"
 blob = "s3"
 lookup = "postgresql"
 directory = "internal"
-in-memory = "postgresql"
+in-memory = "valkey"
 
 # Directory configuration
 [directory.internal]