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/internal/database/db/models.go

@@ -336,6 +336,14 @@ type CoreScheduledJobRun struct {
 	FinishedAt  pgtype.Timestamptz `json:"finished_at"`
 }
 
+type CoreSetting struct {
+	Key       string             `json:"key"`
+	Value     []byte             `json:"value"`
+	Secret    bool               `json:"secret"`
+	UpdatedBy *string            `json:"updated_by"`
+	UpdatedAt pgtype.Timestamptz `json:"updated_at"`
+}
+
 type CoreUser struct {
 	ID            uuid.UUID          `json:"id"`
 	AuthSubject   string             `json:"auth_subject"`