Improve PosixFS configuration defaults and add helpful comments

[michaelstingl]

Background

While testing PosixFS support from PR #125, discovered configuration issues that could be prevented with better defaults and helpful comments in values.yaml.

Proposed Changes

1. Update default mount path

posixfs:
  rootPath: "/mnt/posixfs"  # Change from /var/lib/opencloud/storage to avoid OCM conflicts

2. Add helpful comments with links to docs

storage:
  # Storage mode: "s3" (default) or "posixfs"
  # For PosixFS documentation see: https://docs.opencloud.eu/docs/admin/configuration/storage/storage-posix/
  mode: s3
  
  posixfs:
    # IMPORTANT: Path must be outside /var/lib/opencloud to avoid permission conflicts
    # See: https://docs.opencloud.eu/docs/admin/configuration/storage/storage-posix/
    rootPath: "/mnt/posixfs"
    persistence:
      enabled: true
      size: 30Gi
      # Tip: Can use different storage classes for cost optimization
      # e.g., "ssd" for system data, "sata-large" for user files
      storageClass: "standard"

3. Add short warning in README.md

## Storage Modes

### PosixFS
⚠️ **Important**: PosixFS requires exclusive filesystem access in non-collaborative mode (default).
Do not modify files directly while OpenCloud is running!

For details see: [OpenCloud PosixFS Documentation](https://docs.opencloud.eu/docs/admin/configuration/storage/storage-posix/)

Benefits

• Prevents common deployment errors (OCM permission issue)
• Links to authoritative documentation (no duplication)
• Inline hints for cloud provider optimization
• Keeps helm chart documentation minimal but helpful

[hugoallaine]

I am writing to inform you that with opencloud.storage.mode: posixfs, the description given when running helm install tells me that I am using decomposeds3. I assume this is a bug.

Here is the text:

NAME: opencloud
LAST DEPLOYED: Sun Sep 28 12:27:56 2025
NAMESPACE: opencloud
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
Thank you for installing opencloud.

Your release is named opencloud.

To learn more about the release, try:

  $ helm status opencloud
  $ helm get all opencloud

IMPORTANT: This is a development deployment. For production use, you MUST change the following default credentials:

1. Keycloak Admin: adminUser: admin, adminPassword: admin
2. OpenCloud Admin: adminPassword: admin
3. PostgreSQL: user: keycloak, password: keycloak
4. MinIO: rootUser: opencloud, rootPassword: opencloud-secret-key
5. OnlyOffice Database: sql.dbUser: onlyoffice, sql.dbPass: onlyoffice
6. RabbitMQ: url: amqp://guest:guest@localhost

Using default credentials in production environments poses significant security risks.

The following services have been deployed:

1. OpenCloud (Main Application):
   - Service: opencloud-opencloud
   - Port: 9200
   - Storage Driver: decomposeds3
   - System Storage Driver: decomposed
   - S3 Storage: Not configured
   - S3 Bucket: Not configured