feat(observability): add Prometheus metrics with OpenTelemetry

[faranjit]

• Implement InitMetrics() with Prometheus exporter
• Add shutdown function for graceful cleanup
• Register queue depth gauge in controller
• Add dedicated metrics server on :6162 for worker
• Add comprehensive tests including OTel→Prometheus pipeline test

[codecov]

Codecov Report

❌ Patch coverage is 49.43820% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.78%. Comparing base (e335338) to head (0293030).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/controller/main.go	0.00%	20 Missing ⚠️
cmd/worker/main.go	0.00%	13 Missing ⚠️
internal/store/postgres/queue.go	58.82%	7 Missing ⚠️
internal/controller/server.go	0.00%	3 Missing ⚠️
internal/observability/metrics.go	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
- Coverage   66.77%   65.78%   -1.00%     
==========================================
  Files          32       33       +1     
  Lines        1478     1555      +77     
==========================================
+ Hits          987     1023      +36     
- Misses        426      466      +40     
- Partials       65       66       +1     

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