fix(slack): prevent incorrect deployer shown in deploy notifications

[billyvg]

When GoCD sends a non-@sentry.io approved-by value (e.g., a system
username), getUser() was setting email to undefined, causing findUser()
to build an empty WHERE clause that returned the first user in the
database - often a former employee.

Changes:

• Add isSentryEmail() check before calling getUser() in deploy notifications
• Add guard in findUser() to return empty result when no valid params
• Pass approvedBy as fallback to show raw GoCD username when Slack user
  not found (e.g., "admin has finished deploying" instead of wrong user)

Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.29%. Comparing base (c713794) to head (43c727c).

Files with missing lines	Patch %	Lines
src/brain/gocd/notifyOnGoCDStageEvent/index.ts	80.00%	1 Missing ⚠️
src/utils/db/findUser.ts	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #936   +/-   ##
=======================================
  Coverage   89.28%   89.29%           
=======================================
  Files         123      123           
  Lines        3828     3838   +10     
  Branches      786      790    +4     
=======================================
+ Hits         3418     3427    +9     
- Misses        376      397   +21     
+ Partials       34       14   -20     

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