[V2] Move platform-wide config out of YAML files

[perki]

Cluster-wide settings (domain, hostings, password policy, integrity algorithm, mail templates, etc.) move from per-core YAML to a replicated PlatformDB-backed store. Operators edit once and every core picks up the change; configuration drift between cores becomes detectable instead of silent.

Features:

• Single source of truth for ~15 platform-wide settings; YAML stays for per-core values (ports, paths, secrets) and bootstrap settings (PlatformDB connection, admin access key).
• Encrypted-secrets family: SMTP credentials and SMS provider URLs live in PlatformDB at-rest-encrypted, not scattered across per-core YAML.
• Live drift warnings on boot when a core's local YAML disagrees with the cluster-replicated value.
• Operator lockdown: declare which keys are externally managed (GitOps, Dokku, Cloudflare) and they become read-only in the admin surface.
• Public REST API for read/write with audit trail.

[perki]

Status update: the design for moving platform-wide configuration out of the YAML files is being folded into the operator configuration / admin-panel workstream (a stored, API-exposed platform config with an effective-config view). The feature itself is not yet built, so this issue stays open as the tracker.