Compact /stats-history responses by default

[gglucass]

Description

Compacts /stats-history checkpoint payloads by default so long-running installs do not return ever-growing JSON responses, while keeping history_mode=full for explicit export/debug flows and history_mode=none for rollup-only clients.

Fixes #(issue number): not tracked as a GitHub issue.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• Return compact checkpoint history by default from /stats-history, plus a history_summary block and history_mode={compact,full,none} support.
• Keep dashboard JSON export on the full checkpoint list by requesting history_mode=full.
• Add coverage for compact/full/none history responses and update changelog/wiki docs.

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

$ rtk uvx --with '.[dev]' pytest tests/test_proxy_savings_history.py -vv
12 passed in 11.56s

$ rtk uvx --with '.[dev]' ruff check headroom/proxy/savings_tracker.py headroom/proxy/server.py tests/test_proxy_savings_history.py
All checks passed!

$ rtk uvx --with '.[dev]' mypy headroom
Success: no issues found in 299 source files

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

Screenshots (if applicable)

N/A

Additional Notes

• No matching upstream issue was open for /stats-history growth when I prepared this PR.
• This keeps backward-compatible access to the full retained checkpoint list via history_mode=full; the default change only affects the size of the JSON response payload.

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
headroom/proxy/savings_tracker.py	81.25%	3 Missing and 3 partials ⚠️

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