Metrics and traces for what had none, and alerts on them (#365-#377)

Reviewing what production's traces and metrics leave out turned up
twelve gaps; each is an issue.

Metrics. A Prometheus histogram for ipx and the directory both
(ipx_directory::metrics::Histograms), which ipx's request histogram
(#364) now uses too.
- ipx: each statement's time once it has a connection, by its first
  word, from SeaORM's metric callback; and the wait for one, for
  hand-written SQL in db.rs, measured as the call less the statement
  (the callback runs in the poll that returns the result, so the time
  read after the await is that statement's) (#367). SeaORM's own queries
  are timed but their wait is not seen: a shortcut, noted.
- ipx: counters of what the events say, counted in emit, which every
  event passes once: feed failures by error.type, new items, downloads
  saved and failed, DeadAir's seconds found (#368).
- ipx: the page's list_shown, play_start and stall timings (#369).
- directory: requests by method, route (a route layer puts the matched
  route on the response, or each feed's page was a row) and status;
  feeds checked and failed; snapshots taken (#370).
- ipx-web: Caddy's own metrics on METRICS_BIND (:9180), served apart from
  the port the tunnel reaches. `servers { metrics }`, which Caddy 2.11
  calls deprecated, because the browser tests' Caddy is 2.6 (#371).

Traces.
- A list's sync is list_sync with steps, an OPML upload opml_import
  (#372). Doing so showed sync_group reading every account and its
  subscription to the list once per listed feed; read once now (#377).
- Each command the worker runs, from the socket, the CLI or the page,
  is a command span with its job inside (#373).
- The daemon's start is a trace, from opening the database through the
  catalogue's repairs to "daemon started" (#374).
- The directory's lines carry trace_id and span_id: WithTrace moves to
  ipx_directory::otel and both use it (#375).
- A tap or key is its trace's root span (#376). The page names a root
  span id in each request's traceparent, where the parent was random and
  pointed at nothing, and reports the tap (ids, start, its last answer)
  when the next begins or the page is hidden. opentelemetry 0.33 gives a
  span builder no way to take ids, so the shared tracer provider has an
  id generator that takes given ids inside with_ids, on one thread.

Alerts. grafana/alerts.py, Grafana alerting provisioning beside the
dashboard (#365): ipx, the directory or ipx-web down, scans stopped, a
job over 30 minutes, the pool full, 5xx, 502s, Jev refused, Signals
stopped, the directory's refresher stopped, Tower's load and memory, the
monitoring down, Tempo not reading its blocks, iPX's lines not reaching
Loki. No contact point yet: firing shows in Grafana. Prometheus now
scrapes Tempo, Loki, Alloy and itself (#366), in the monitoring project's
config, not this repository's.

The dashboard has database and page-timing panels; the prod check knows
the new metrics, spans and alerts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/Caddyfile

@@ -6,12 +6,19 @@
 #                 OTEL_EXPORTER_OTLP_PROTOCOL, OTEL_SERVICE_NAME), continuing the page's traceparent
 #                 and passing it to ipx, so one trace runs browser, Caddy, ipx (#213). Off: none,
 #                 and no export failing every few seconds where there is nothing to send to.
+#   METRICS_BIND  where to serve Caddy's metrics for Prometheus  (:9180); keep it off the tunnel
 #   TUNNEL_PEER   the one address whose Cf-Access-* headers are passed on: where cloudflared's
 #                 requests come from. Nobody else's are, so nobody else can claim to be signed in
 #                 by Cloudflare. Unset passes none.
 {
 	admin off
 	auto_https off
+	# Requests, their times and upstream errors as Prometheus metrics, served below (#371).
+	# Nested in servers, which Caddy 2.11 calls deprecated, for the browser tests' Caddy, 2.6,
+	# which knows no other way; the global `metrics` option once both are 2.8 or later.
+	servers {
+		metrics
+	}
 	# Everything else Caddy logs, its errors among them, which carry the request too: the same
 	# filter as the access log's below.
 	log default {
@@ -28,6 +35,11 @@
 	}
 }
 
+# Caddy's own metrics (#371), on a port of their own: the tunnel reaches only WEB_BIND's.
+http://{$METRICS_BIND::9180} {
+	metrics
+}
+
 (tracing-on) {
 	tracing {
 		# Named for the request, as ipx names its own (#286): one name for all left every span in