feat(web-app-gitea): add optional Redis integration for caching, sessions, and queues

This update introduces conditional Redis support for Gitea, allowing connection
to either a local or centralized Redis instance depending on configuration.
Includes resource limits for the Redis service and corresponding environment
variables for cache, session, and queue backends.

Reference: ChatGPT conversation on centralized vs per-app Redis architecture (2025-10-15).
https://chatgpt.com/share/68ef5930-49c8-800f-b6b8-069e6fefda01

roles/web-app-gitea/templates/env.j2

@@ -11,7 +11,7 @@ USER_GID=1000
 
 # Logging configuration
 GITEA__log__MODE=console
-GITEA__log__LEVEL={% if MODE_DEBUG | bool %}Debug{% else %}Info{% endif %}    
+GITEA__log__LEVEL={% if MODE_DEBUG | bool %}Debug{% else %}Info{% endif %}
 
 # Database
 DB_TYPE=mysql
@@ -20,6 +20,28 @@ DB_NAME={{ database_name }}
 DB_USER={{ database_username }}
 DB_PASSWD={{ database_password }}
 
+
+{% if GITEA_REDIS_ENABLED | bool %}
+# ------------------------------------------------
+# Redis Configuration for Gitea
+# ------------------------------------------------
+# @see https://docs.gitea.com/administration/config-cheat-sheet#cache-cache
+
+GITEA__cache__ENABLED=true
+GITEA__cache__ADAPTER=redis
+# use a different Redis DB index than oauth2-proxy
+GITEA__cache__HOST=redis://{{ GITEA_REDIS_ADDRESS }}/1
+
+# Store sessions in Redis (instead of the internal DB)
+GITEA__session__PROVIDER=redis
+GITEA__session__PROVIDER_CONFIG=network=tcp,addr={{ GITEA_REDIS_ADDRESS }},db=2,pool_size=100,idle_timeout=180
+
+# Use Redis for background task queues
+GITEA__queue__TYPE=redis
+GITEA__queue__CONN_STR=redis://{{ GITEA_REDIS_ADDRESS }}/3
+{% endif %}
+
+
 # SSH
 SSH_PORT={{ports.public.ssh[application_id]}}
 SSH_LISTEN_PORT=22