Fix EmailFormProcessor ignoring block settings

[ericof]

What

Fixes the EmailFormProcessor silently ignoring the form block's configured
settings, and moves the mail-template storage to a JSONField so template
selection works end to end.

The processor read several block fields under the wrong keys — most visibly
email_template instead of mail_template — so the selected mail template,
admin info, mail header/footer and sender/recipient addresses were dropped and
the defaults used instead.

Changes

Fix + refactor (0d24af5)

• Type the block payload (SchemaFormBlock) and the submission context, so a
  wrong key is visible rather than silently defaulted.
• Extract the email helpers (substitute_variables, format_property,
  addresses_from_block, template lookup, mailhost check) into utils.email;
  the processor delegates to them.
• Resolve sender/recipient/bcc addresses once, up front, and pass them through
  the submission context.
• Move the mail-templates registry setting to a JSONField
  (schemaform.mail_templates_json), deprecating schemaform.mail_templates,
  with an upgrade step (profile 1000 → 1001) migrating existing templates.
• Add unit tests for the utils.email helpers.

Testing profile (74fd676)

• Replace the imperative registry setup in the test layer with a declarative
  plone.formblock.testing:testing GenericSetup profile that configures the
  mail settings and installs the captcha add-ons (norobots, hcaptcha,
  recaptcha). Captcha vocabulary test updated for the now-available norobots
  adapter.

Testing

• make test — full backend suite green (146 passed).
• ruff check / ruff format clean.

Related issue

Refs #19

Note: the httpHeaders portion of #19 is a frontend concern and is tracked
separately, so this PR does not close the issue.

[pbauer]

Works fine now. Thanks!