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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apps/backend/db/queries/email_log.sql

@@ -0,0 +1,10 @@
+-- name: InsertEmailLog :one
+INSERT INTO core.email_log (project_id, template, to_email, subject, status, error)
+VALUES ($1, $2, $3, $4, $5, $6)
+RETURNING *;
+
+-- name: ListEmailLog :many
+SELECT * FROM core.email_log
+WHERE project_id = $1
+ORDER BY created_at DESC
+LIMIT $2;