feat: HA scheduler leadership + templated email log

HA scheduler: the job scheduler and alert evaluator hold session-scoped
PostgreSQL advisory locks on pinned pool connections — replicas scan for
due work exactly once, failover verified live with two replicas. Manual
and hook-triggered runs stay available on any replica.

Email surface: mailer renders registered templates (invitation); every
send is logged to core.email_log with status and error text. New
GET /projects/:id/emails endpoint; Members page shows an Email log card.

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

apps/backend/internal/database/db/models.go

@@ -243,6 +243,17 @@ type CoreDocument struct {
 	UpdatedAt       pgtype.Timestamptz `json:"updated_at"`
 }
 
+type CoreEmailLog struct {
+	ID        uuid.UUID          `json:"id"`
+	ProjectID pgtype.UUID        `json:"project_id"`
+	Template  string             `json:"template"`
+	ToEmail   string             `json:"to_email"`
+	Subject   string             `json:"subject"`
+	Status    string             `json:"status"`
+	Error     string             `json:"error"`
+	CreatedAt pgtype.Timestamptz `json:"created_at"`
+}
+
 type CoreEvent struct {
 	ID          int64              `json:"id"`
 	ProjectID   uuid.UUID          `json:"project_id"`