feat: instance administration, bootstrap admin, and managed-tier OAuth gate

Self-hosted instances are now email/password only — OAuth providers
(GitHub/Google/Discord/Microsoft) activate solely when PRIMORA_MANAGED=true,
reserved for the future hosted tier.

Sign-up lifecycle: the first public sign-up becomes the instance admin; after
bootstrap, sign-up closes by default. A better-auth databaseHook gates only
/sign-up/email, so admin-created users and OAuth flows bypass it. The login
page reads GET /instance/public to hide the sign-up tab, show the bootstrap
notice, and render only the providers the instance offers.

Instance settings live in core.settings (migration 00010) and resolve
in-app > env > built-in default, cached 15s. Platform admins (better-auth
user.role) manage sign-up, mail transport, and rate limits via
GET/PUT/DELETE /instance/settings or the new Settings → Instance tab. Secret
values are AES-256-GCM encrypted at rest with PRIMORA_ENCRYPTION_KEY, never
returned by the API, and decrypted by the auth service at send time.

Auth service reads the same settings table for the sign-up gate and mail
config, so in-app changes take effect without a restart.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/db/queries/settings.sql

@@ -0,0 +1,18 @@
+-- name: GetSetting :one
+SELECT key, value, secret, updated_at FROM core.settings WHERE key = $1;
+
+-- name: ListSettings :many
+SELECT key, value, secret, updated_at FROM core.settings ORDER BY key;
+
+-- name: UpsertSetting :one
+INSERT INTO core.settings (key, value, secret, updated_by)
+VALUES ($1, $2::jsonb, $3, $4)
+ON CONFLICT (key) DO UPDATE
+SET value = EXCLUDED.value,
+    secret = EXCLUDED.secret,
+    updated_by = EXCLUDED.updated_by,
+    updated_at = NOW()
+RETURNING *;
+
+-- name: DeleteSetting :exec
+DELETE FROM core.settings WHERE key = $1;