Add Nextcloud cloud storage role with split Redis caching strategy

## New Features

- **Nextcloud Role**: Complete cloud storage deployment using Podman Quadlet
  - FPM variant with Caddy reverse proxy and FastCGI
  - PostgreSQL database via Unix socket
  - Valkey/Redis for app-level caching and file locking
  - Automatic HTTPS with Let's Encrypt via Caddy
  - Dual-root pattern: Caddy serves static assets, FPM handles PHP

- **Split Caching Strategy**: Redis caching WITHOUT Redis sessions
  - Custom redis.config.php template for app-level caching only
  - File-based PHP sessions for stability (avoids session lock issues)
  - Prevents cascading failures from session lock contention
  - Documented in role README with detailed rationale

## Infrastructure Updates

- **Socket Permissions**: Update PostgreSQL and Valkey to mode 777
  - Required for containers that switch users (root → www-data)
  - Nextcloud container loses supplementary groups on user switch
  - Security maintained via password authentication (scram-sha-256, requirepass)
  - Documented socket permission architecture in docs/

- **PostgreSQL**: Export client group GID as fact for dependent roles
- **Valkey**: Export client group GID as fact, update socket fix service

## Documentation

- New: docs/socket-permissions-architecture.md
  - Explains 777 vs 770 socket permission trade-offs
  - Documents why group-based access doesn't work for user-switching containers
  - Provides TCP alternative for stricter security requirements

- Updated: All role READMEs with socket permission notes
- New: Nextcloud README with comprehensive deployment, troubleshooting, and Redis architecture documentation

## Configuration

- host_vars: Add Nextcloud vault variables and configuration
- site.yml: Include Nextcloud role in main playbook

## Technical Details

**Why disable Redis sessions?**

The official Nextcloud container enables Redis session handling via REDIS_HOST env var,
which causes severe performance issues:

1. Session lock contention under high concurrency (browser parallel asset requests)
2. Infinite lock retries (default lock_retries=-1) blocking FPM workers
3. Timeout orphaning: reverse proxy kills connection, worker keeps lock
4. Worker pool exhaustion: all 5 default workers blocked on same session lock
5. Cascading failure: new requests queue, more timeouts, more orphaned locks

Solution: Use file-based sessions (reliable, fast for single-server) while keeping
Redis for distributed cache and transactional file locking via custom config file.

This provides optimal performance without the complexity of Redis session debugging.

Tested: Fresh deployment on arch-vps (69.62.119.31)
Domain: https://cloud.jnss.me/

roles/nextcloud/meta/main.yml

@@ -0,0 +1,17 @@
+---
+dependencies:
+  - role: postgresql
+  - role: valkey
+  - role: caddy
+  - role: podman
+
+galaxy_info:
+  author: rick-infra
+  description: Nextcloud cloud storage with FPM, PostgreSQL, and Valkey
+  company: jnss.me
+  license: MIT
+  min_ansible_version: "2.9"
+  platforms:
+    - name: ArchLinux
+      versions:
+        - all