server: every dns listener shares one budget of 32 queries, stream listeners take 16 connections, the task pool is capped from those bounds, and work it cannot start is counted by site

CHANGELOG.md

@@ -19,6 +19,8 @@ Sections are written by hand. Nothing here is generated from commit messages: th
 - **Blocklists take less memory, and a refresh peaks lower.** A list that names a domain both on its own and with its subdomains, as Adblock-style lists do for every entry, now stores that name once instead of twice. A reload reads one list's compiled files at a time instead of all of them at once, and the startup pass no longer rebuilds the blocklists when no list was due for a refresh.
 - **The query-history buffer defaults to 1024 queries** instead of 10000, under 2 MiB instead of about 17. It only holds queries that arrive while a write is in progress.
 - **A live query stream takes memory only while someone watches it.** Each of the 32 possible streams reserved its buffer at startup, about 3.6 MiB in all. A stream that cannot get its buffer is refused with its own message and counted in `nxdns_live_stream_allocation_failures_total`.
+- **All DNS listeners share one budget of 32 queries at a time**, about 2.4 MiB, where each UDP and TCP listener used to reserve room for 64 of its own, about 27 MiB across the four. The Pi's busiest moment over 37 hours was 27 queries at once. A query beyond the budget is dropped and counted rather than queued, so a device retries it: watch `nxdns_udp_server_dropped_no_slot_total` and the stream listeners' new `dropped_no_permit_total`. TCP, DoT and DoH each accept 16 connections at a time instead of 64, and the DoT and DoH listeners now report `rejected_at_capacity_total` too.
+- **The server runs a bounded number of threads**, sized from those caps, instead of starting a new one whenever all were busy. Work that could not get a thread used to fail as a generic error or not be counted at all; `/metrics` now counts it as `nxdns_concurrency_unavailable_total`, by where it happened.
 - **Each database connection keeps a page cache sized for its job** instead of SQLite's 2 MiB default on all seven: 256 KiB for the configuration database and for retention, 1 MiB for the query-history writer and reader.
 - **The documentation is written for people running nxdns.** It starts from Get started, which downloads a release instead of building from source, and drops the API reference, the troubleshooting page and the performance pages. It is published at <https://nxdns.sh/docs/>, and `INSTALL.md` in the tarball links there.