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/db/migrations/00014_email_log.sql

@@ -0,0 +1,16 @@
+-- +goose Up
+CREATE TABLE core.email_log (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  project_id UUID REFERENCES core.projects(id) ON DELETE CASCADE,
+  template TEXT NOT NULL,
+  to_email TEXT NOT NULL,
+  subject TEXT NOT NULL,
+  status TEXT NOT NULL CHECK (status IN ('sent','failed')),
+  error TEXT NOT NULL DEFAULT '',
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_email_log_project ON core.email_log(project_id, created_at DESC);
+
+-- +goose Down
+DROP TABLE IF EXISTS core.email_log;