homarr: add dashboard stack with PocketID OIDC

Created by: jedmund

Summary

• Adds Homarr (ghcr.io/homarr-labs/homarr) as a new stack at dash.atelier.house
• Configures OIDC authentication via PocketID (https://id.atelier.house)
• Uses a bind-mounted ./data directory for SQLite persistence (no external volume needed)
• Registers homarr host group in inventory targeting nuc-mini

Vault secrets required before deploying

• homarr_oidc_client_id — client ID registered in PocketID
• homarr_oidc_client_secret — client secret from PocketID
• homarr_secret_encryption_key — 64-character hex string for session encryption

Test plan

• Register a new OIDC client in PocketID for Homarr (dash.atelier.house)
• Add vault secrets for the three vars above
• Run deploy/homarr.yml and verify the container starts healthy
• Confirm redirect to PocketID login and successful auth callback
• Verify admin/owner group mappings work as expected