fix(history): implement thread-safe connection pool for SQLite to fix resource leaks

[mhajder]

Refactors SyncHistory to use a queue.Queue connection pool instead of threading.local().

Previously, connections tied to threading.local() in a background worker pool would remain open until the thread died, leading to a steady leak of open file descriptors and locked connections.

Changes:

• Replaced threading.local() with a bounded queue.Queue pool (size=5) of sqlite3.Connection objects configured with check_same_thread=False.
• Safely wrapped the initial schema connection in a try...finally block to ensure close() is called, fixing a silent file lock leak.
• Added PRAGMA synchronous=NORMAL alongside journal_mode=WAL for drastically improved concurrent write performance without sacrificing durability.
• Updated all database operations to borrow from the pool using a context manager, allowing true multi-threaded concurrency (concurrent readers/writers via WAL) while strictly bounding the total number of open database connections.
• Updated close() to gracefully drain the queue and explicitly terminate all pooled connections.