[Tile-serving] [openstreetmap/mod_tile] Configure renderd request queue limits (PR #498)

[Darafei Praliaskouski]

## Summary

This replaces the old hard-coded dirty queue limit approach from https://github.com/openstreetmap/mod_tile/pull/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

```sh
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.

You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/mod_tile/pull/498

-- Commit Summary --

  * feat(renderd): configure request queue limits

-- File Changes --

    M docs/man/renderd.conf.5 (15)
    A docs/renderd-queue-tuning.md (68)
    M etc/renderd/renderd.conf (2)
    M etc/renderd/renderd.conf.examples (4)
    M etc/renderd/renderd.conf.in (2)
    M includes/render_config.h (9)
    M includes/renderd.h (2)
    M includes/request_queue.h (3)
    M src/renderd.c (2)
    M src/renderd_config.c (16)
    M src/request_queue.c (53)
    M tests/gen_tile_test.cpp (44)
    M tests/renderd_config_test.cpp (47)
    M tests/renderd_config_test_helper.cpp (12)

-- Patch Links --

https://github.com/openstreetmap/mod_tile/pull/498.patchhttps://github.com/openstreetmap/mod_tile/pull/498.diff
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