Align Archera signup URL in email templates with CLI/web canonical #48

Context

PR LeanerCloud/cloud-commitments-cli#1280 ("feat(archera): soft-pitch insurance after CLI purchase + update web signup link") updated the canonical Archera signup URL across the CLI and web frontend to https://www.archera.ai/cudly, but explicitly left the email templates out of scope.

The email approval / confirmation templates still embed the prior URL https://archera.ai/signup?mode=cudly:

• internal/email/templates.go — template literals
• internal/email/template_renderers_test.go — assertions pinning the old URL

Since the URL is the canonical attribution destination across CLI, web modal, and (soon) emails, leaving the email templates on the old URL means users who click the link in their approval / confirmation email land somewhere different from where the CLI and web tell them to go.

Acceptance criteria

• internal/email/templates.go: replace the embedded URL with https://archera.ai/cudly.
• Define a shared Go constant (e.g. exposed from pkg/archera or wherever fits) so the CLI (cmd/multi_service_stats.go), email templates, and any future server-side code reference one source of truth, mirroring how the frontend exports ARCHERA_SIGNUP_URL from frontend/src/archera.ts.
• internal/email/template_renderers_test.go: update assertions to pin the new literal URL.
• No silent fallbacks: tests should pin the literal URL (not the constant) at least once per template family, so a future constant-change to a wrong destination is caught (mirroring the CodeRabbit nit addressed in feat(archera): soft-pitch insurance after CLI purchase + update web signup link cloud-commitments-cli#1280).
• Confirm both partnership disclosures (non-gating + sponsorship — see project_archera_partnership memory) remain visible in the email body alongside the new URL.

Related

• PR feat(archera): soft-pitch insurance after CLI purchase + update web signup link cloud-commitments-cli#1280 (out-of-scope note in the description)
• Issue feat(frontend): Archera Insurance CTA in Purchase + Plan-creation modals + education pages cloud-commitments-cli#314 (Archera integration scope)