Add a generic admin-editable config layer (epic 4a): system_config KV table
(migration 0021), a sysconfig registry that is the single source of truth for
DB-overridable keys (type/group/default/bounds/secret) + a DB-override-or-env
resolver, and admin endpoints (GET/PATCH/DELETE /api/admin/config + a test-email
probe). Secrets are Fernet-encrypted at rest (TOKEN_ENCRYPTION_KEY); without it
they stay env-only. First group wired end-to-end: Email/SMTP — email.py now
reads host/port/user/from/password via the resolver (own session, since email
is sent from background tasks).
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