feat: run queries on their own Tokio runtime

[robinskil]

What changes does this PR make?

Queries run on their own Tokio runtime. The API runs on a second one.

Why do you make these changes?

One runtime served the API and ran every query. A scan holds a worker until a partition yields. One long query took every thread. The API stopped answering.

How do you make these changes?

main builds two runtimes. The new query_executor module in beacon-core holds the query handle. Planning and execution run there. A bounded channel moves batches to the caller. A disconnect cancels the query. A panic reaches the client as an error.

Does this PR change a public interface or a configuration?

New variable BEACON_API_THREADS, default 4. Server::open takes the query runtime handle. Not breaking.

How do you test these changes?

Seven unit tests cover the executor. One config test covers thread counts.

cargo test -p beacon-core -p beacon-server -p beacon-server-config --tests

Four known Windows-only tests fail, as on main.

Checklist

• Title describes the change.
• Changed lines pass rustfmt.
• No new clippy warnings.
• Tests pass.
• New code has tests.
• Docs and CHANGELOG show the change.
• No unrelated changes.

Was this patch authored or co-authored using generative AI tooling?

Yes. Assisted-by: Claude:claude-fable-5-1

[robinskil]

Closed by the branch rename. The same change continues in #476.

[codecov]

Codecov Report

❌ Patch coverage is 87.54864% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.23%. Comparing base (1d726a7) to head (fa3ee2e).

Files with missing lines	Patch %	Lines
beacon-server/beacon-server/src/main.rs	0.00%	25 Missing ⚠️
beacon-db/beacon-core/src/query_executor.rs	95.67%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #475      +/-   ##
==========================================
+ Coverage   83.20%   83.23%   +0.02%     
==========================================
  Files         372      373       +1     
  Lines       62250    62526     +276     
==========================================
+ Hits        51798    52044     +246     
- Misses      10452    10482      +30     

Files with missing lines	Coverage Δ
beacon-db/beacon-core/src/runtime.rs	97.21% <100.00%> (-0.17%)	⬇️
beacon-db/beacon-core/src/runtime_builder.rs	92.23% <100.00%> (+0.01%)	⬆️
beacon-server/beacon-server-config/src/lib.rs	94.78% <100.00%> (+0.32%)	⬆️
beacon-server/beacon-server/src/server/mod.rs	84.48% <100.00%> (+0.08%)	⬆️
beacon-db/beacon-core/src/query_executor.rs	95.67% <95.67%> (ø)
beacon-server/beacon-server/src/main.rs	19.62% <0.00%> (-4.52%)	⬇️

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