server: set all four http.Server socket timeouts (#118)

The http.Server was built with only ReadHeaderTimeout set; the other three
timeouts were zero, which in net/http means no limit, so a peer could hold a
connection open past the header phase, responses had no write deadline, and
keep-alive connections were never reaped. ReadTimeout 15s, WriteTimeout 75s,
and IdleTimeout 120s now join ReadHeaderTimeout 10s as named constants.
WriteTimeout must stay above the 60s chimw.Timeout handler budget, because
net/http arms the write deadline once request headers are read; a test fails
the build if either number moves alone. The server literal moved into
newHTTPServer so the configuration can be asserted without binding a socket
(closes #99)

Model: opus-5

README.md

@@ -182,6 +182,27 @@ dnswatcher exposes a lightweight HTTP API for operational visibility:
 | `GET /api/v1/status`                  | Current monitoring state       |
 | `GET /metrics`                        | Prometheus metrics (optional)  |
 
+#### Server timeouts
+
+The HTTP server sets all four socket-level timeouts. These are compile-time
+constants in `internal/server/server.go`, not configurable via environment
+variables.
+
+| Timeout             | Value | Purpose                                       |
+|---------------------|-------|-----------------------------------------------|
+| `ReadHeaderTimeout` | 10s   | Bounds the request header read (slowloris)    |
+| `ReadTimeout`       | 15s   | Bounds the whole request read, headers + body |
+| `WriteTimeout`      | 75s   | Bounds handler execution plus response flush  |
+| `IdleTimeout`       | 120s  | Reaps idle keep-alive connections             |
+
+These are distinct from the 60s per-request handler budget applied by
+`chimw.Timeout` in `internal/server/routes.go`, which cancels the request
+context but does not touch the socket. `WriteTimeout` is deliberately
+larger than that budget: the write deadline is armed once request headers
+are read, so a smaller value would sever the connection before a handler
+using its full budget could respond. `IdleTimeout` exceeds common
+Prometheus scrape intervals so the scraper reuses its connection.
+
 ---
 
 ## Architecture