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feat: background template reloading p2 - mailer template cache & interface refactor (#2150)

**Summary**
This PR builds on
[auth#2148](https://github.com/supabase/auth/pull/2148) to deliver phase
2, a robust template cache plus a simplified mail client interface.
Templates are now rendered once per type, cached with controlled
refresh, and then delivered via clients that only send pre-rendered
subject/body. This reduces latency and removes templating logic from
SMTP clients while preserving current behavior. It will also pave the
way for alternative Mailer implementations.

**Template cache + interface refactor**
* Replace `TemplateMailer` with `templatemailer.Mailer` that owns all
templating and a shared cache. Entries have a TTL and a shorter “recheck
interval”; on fetch/parse errors we serve the last good or fall back to
built-in defaults.
* Centralize default subjects/bodies and validate them at init. Typed
template keys are: `invite`, `confirmation, recovery`, `email_change`,
`magic_link`, `reauthentication`.
* Move headers logic into the mailer; keep `$messageType` substitution
and the legacy “reauthenticate” header name for compatibility.
* Change `mailer.Client.Mail` to `(ctx, to, subject, body, headers,
typ)` and update all clients accordingly (SMTP/Noop/Validate/Tasks).
* Persist a single mailer instance in `API`; add `WithMailer` for DI;
remove on-demand construction.

---------

Co-authored-by: Chris Stockton <chris.stockton@supabase.io>