Upgrade EmailService for changes in MailCheck API

[emmiegit]

The MailCheck API has made some changes to its response shape, along with a lower unauthenticated rate limit.

This PR addresses this and a few other issues with EmailService:

• Updates the response structures used to understand the API. This adds several more fields, and also avoids the now-deprecated alias field.
• Stores the whole MailCheck response in the database for future reference. Presently only the alias information was stored.
• Separates email verification time (when the user confirms that the email belongs to them) from email validation time (when we ran it through the MailCheck API).
• Optionally allows setting the MAILCHECK_API_KEY environment variable for using a non-anonymous requests.
• Bypass the email verification check for site users. (Not relevant and wastes our rate limit).
• Doesn't request on empty email strings.
• Adds rustdoc comments for the MailCheck API fields.
• Bumps version to 2026.3.6.

I tested local user creation, both for valid and invalid emails.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 109 lines in your changes missing coverage. Please review.
✅ Project coverage is 2.27%. Comparing base (dfc6c81) to head (d8a9d71).
⚠️ Report is 21 commits behind head on develop.

Files with missing lines	Patch %	Lines
deepwell/src/services/email/service.rs	0.00%	59 Missing ⚠️
deepwell/src/services/user/service.rs	0.00%	28 Missing ⚠️
deepwell/src/api.rs	0.00%	14 Missing ⚠️
deepwell/src/config/secrets.rs	0.00%	7 Missing ⚠️
deepwell/src/endpoints/email.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #2736      +/-   ##
==========================================
- Coverage     2.29%   2.27%   -0.02%     
==========================================
  Files          171     170       -1     
  Lines        10287   10356      +69     
==========================================
  Hits           236     236              
- Misses       10051   10120      +69     

Files with missing lines	Coverage Δ
deepwell/src/models/user.rs	0.00% <ø> (ø)
deepwell/src/services/site/service.rs	0.00% <ø> (ø)
deepwell/src/endpoints/email.rs	0.00% <0.00%> (ø)
deepwell/src/config/secrets.rs	0.00% <0.00%> (ø)
deepwell/src/api.rs	0.00% <0.00%> (ø)
deepwell/src/services/user/service.rs	0.00% <0.00%> (ø)
deepwell/src/services/email/service.rs	0.00% <0.00%> (ø)

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