feat: Add email notification when a part is changed

[matmair]

This PR:

• re-adds the subscription buttons for parts and part categories (was not re-implemented when porting from CUI)
• adds a built-in plugin that optionally notifies on part events

Closes #7834

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[codecov]

Codecov Report

Attention: Patch coverage is 64.58333% with 17 lines in your changes missing coverage. Please review.

Project coverage is 86.14%. Comparing base (94ded91) to head (2000b83).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
...end/src/components/buttons/StarredToggleButton.tsx	38.46%	6 Missing and 2 partials ⚠️
...e/plugin/builtin/integration/part_notifications.py	77.41%	7 Missing ⚠️
src/frontend/src/pages/part/CategoryDetail.tsx	0.00%	1 Missing ⚠️
src/frontend/src/pages/part/PartDetail.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9275      +/-   ##
==========================================
- Coverage   86.18%   86.14%   -0.05%     
==========================================
  Files        1199     1201       +2     
  Lines       52572    52619      +47     
  Branches     2216     2220       +4     
==========================================
+ Hits        45311    45328      +17     
- Misses       6699     6720      +21     
- Partials      562      571       +9     

Flag	Coverage Δ
backend	88.07% <78.78%> (-0.01%)	⬇️
pui	69.81% <33.33%> (-0.21%)	⬇️

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