Refactor proxycache server: thread-safety, atomic writes, and deduplication

[mamico]

Description

This PR refactors the RSS proxycache server () to address thread-safety, concurrency, reliability, and security issues identified during code review.

Key Improvements

• Thread Safety & Race Conditions: Added to synchronize access to and background thread states.
• Background Thread Deduplication: Implemented to ensure at most one background refresh thread runs per URL, preventing thread leaks.
• Atomic Cache File Writes: Implemented using temporary files and to prevent corrupted JSON reads during concurrent client requests.
• Non-blocking Server: Upgraded to so that slow remote feed requests do not block other clients.
• SSRF & URL Validation: Added check to validate URL scheme and prevent basic SSRF probing against internal endpoints.
• Docstring & Usage Preserved: Retained all original docstrings, including usage examples and notes.
• Unit Testing: Added unit tests in covering cache path generation, atomic writing, URL validation, thread deduplication, and error handling.
• Changelog: Updated .

[coveralls]

Coverage Report for CI Build 36587920022

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Coverage: 70.968%

Details

• Patch coverage: 62 uncovered changes across 2 files (50 of 112 lines covered, 44.64%).

Uncovered Changes

File	Changed	Covered	%
src/redturtle/rssservice/proxycacheserver/main.py	107	46	42.99%
src/redturtle/rssservice/rss_mixer.py	4	3	75.0%
Total (3 files)	112	50	44.64%

Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	434
Covered Lines:	308
Line Coverage:	70.97%
Coverage Strength:	2.12 hits per line

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💛 - Coveralls