Configure renderd request queue limits

[Komzpa]

Summary

This replaces the old hard-coded dirty queue limit approach from #152 with configurable renderd.conf queue limits.

• add request_queue_limit and dirty_queue_limit options to the [renderd] section
• preserve the previous compiled-in defaults for metatile and non-metatile builds
• size the queue de-duplication index from configured queue capacity
• document queue behavior in the manpage and add an operator tuning/service manual

Context

The old PR tried to make the dirty queue much larger globally. The review thread pointed out that this is deployment-specific, can make stale background work accumulate, and does not solve queue fairness. This version keeps the default behavior unchanged and lets operators opt into larger queues with rollout guidance.

Testing

cmake --build build --target gen_tile_test render_list renderd_config_test renderd_config_test_helper -j2
ctest --test-dir build -R '^(gen_tile_test|renderd_config_test)$' --output-on-failure

Both targeted tests pass locally.