feat: add optional Redis caching layer for API endpoints

Add Redis-backed response caching for read-heavy API endpoints (nodes,
advertisements, messages, channels, dashboard, profiles) with configurable
TTL, key prefix isolation, and graceful fallback when Redis is unavailable.

New files:
- common/redis.py: CacheBackend, NullCache, RedisCacheBackend
- api/cache.py: @cached decorator, sorted_query_string helper
- tests/test_api/test_cache.py: 23 unit tests

Changes:
- pyproject.toml: add redis[hiredis] dependency
- common/config.py: 8 Redis settings on APISettings
- api/cli.py: Redis Click options + startup banner
- api/app.py: Redis lifespan init/cleanup, X-Cache middleware, health check
- 6 route files: apply @cached decorator to list endpoints
- docker-compose.yml: Redis service (cache profile), env vars
- docker-compose.dev.yml: Redis port exposure
- .env.example, README.md, AGENTS.md, docs/upgrading.md: documentation

Redis is disabled by default (REDIS_ENABLED=false). Enable with
--profile cache and REDIS_ENABLED=true.

pyproject.toml

@@ -45,6 +45,7 @@ dependencies = [
     "markdown>=3.5.0",
     "prometheus-client>=0.20.0",
     "meshcoredecoder>=0.3.2",
+    "redis[hiredis]>=5.0.0",
 ]
 
 [project.optional-dependencies]
@@ -122,6 +123,7 @@ module = [
     "prometheus_client.*",
     "meshcoredecoder.*",
     "authlib.*",
+    "redis.*",
 ]
 ignore_missing_imports = true