Resolve "Beacon async crashes with mixed slow/fast concurrent requests"

Improves Beacon stability under concurrent heavy query load by adding a semaphore with timeout to the request handlers.

Changes

• Add asyncio.Semaphore to limit concurrent heavy queries (2+ filters) and protect the connection pool
• Queries with 0 or 1 filter bypass the semaphore entirely (is_heavy_query check)
• Add HEAVY_Q_TIMEOUT and MAX_CONCURRENT_HEAVY_QUERIES in confTemplate.py for easy configuration
• If a heavy query waits more than HEAVY_Q_TIMEOUT seconds in the semaphore queue, Beacon returns 503 Service Unavailable instead of letting the client timeout silently

Closes #19 (closed)