🔒 [security fix] Fix Unbounded In-Memory Job Storage Leading to Resource Exhaustion

[chottokun]

🎯 What: Unbounded In-Memory Job Storage Leading to Resource Exhaustion

⚠️ Risk: The global in-memory job store (jobs) could grow unbounded as background OCR jobs are created. An attacker could exploit this by spawning a large number of asynchronous jobs, causing high memory usage and eventually triggering an Out of Memory (OOM) crash, leading to a Denial of Service (DoS).

🛡️ Solution: Updated the InMemoryJobStore class to be bounded and thread-safe:

1. Added max_size (default: 1000) and ttl_seconds (default: 3600.0s) parameters, configurable via env variables (JOB_STORE_MAX_SIZE, JOB_STORE_TTL_SECONDS).
2. Implemented Least Recently Used (LRU) eviction when the job store size reaches the maximum capacity.
3. Implemented Time-To-Live (TTL) based eviction upon accesses (get/set/exists calls) to purge expired jobs.
4. Added threading.Lock to ensure job store queries and updates are atomic and thread-safe.
5. Added unit, integration, and concurrency tests in tests/test_job_store_limits.py verifying all eviction and synchronization behaviors.

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PR created automatically by Jules for task 17267331644983903605 started by @chottokun

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