Reload job functions when --watch detects a file change

[Cherith1222]

--watch restarted the same worker, so job functions imported at startup kept running after a file change. The settings module is re-imported and a new worker is started.

Fixes #429.

Cherith reviewed this diff and confirmed it with the person who helped draft it.
Drafted with Cursor. Model: Grok 4.7.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 70.27027% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
arq/cli.py	70.27%	7 Missing and 4 partials ⚠️
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@@            Coverage Diff             @@
##             main     #535      +/-   ##
==========================================
- Coverage   96.27%   94.99%   -1.29%     
==========================================
  Files          11       11              
  Lines        1074     1119      +45     
  Branches      209      152      -57     
==========================================
+ Hits         1034     1063      +29     
- Misses         19       31      +12     
- Partials       21       25       +4     

Files with missing lines	Coverage Δ
arq/cli.py	87.35% <70.27%> (-12.65%)	⬇️

... and 1 file with indirect coverage changes

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