chore(techdebt): replace runtime TODO/FIXME with tracked markers

[SaitejaKommi]

Summary

This PR replaces ambiguous TODO / FIXME markers in selected runtime paths with explicit TECHDEBT(SAL-TECHDEBT-xxx) references.

It also adds runtime-techdebt.md as a centralized mapping index for those identifiers, improving visibility and planning while keeping behavior unchanged.

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Related Issue

Fixes #390

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What Changed

• Replaced untracked TODO / FIXME comments with structured technical debt markers
• Standardized debt annotations across backend and frontend runtime paths
• Added runtime-techdebt.md as the central tracking document
• Improved maintainability and long-term prioritization visibility

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Affected Files

• facets.py
• charts.py
• query.py
• query_transformers.py
• search-suggester.ts
• runtime-techdebt.md

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Testing

Full local gate executed successfully:

• Backend unit tests ✅
• Backend integration tests ✅
• Frontend build and tests (dev) ✅
• Frontend build and tests (prod) ✅

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Notes

This is a documentation and maintainability improvement with no runtime or API behavior changes.