Prometheus metrics for the daemon's state between requests (#194)

Logs and traces show requests, scans and downloads; nothing showed what
the daemon holds meanwhile. /metrics, behind sign-in, gives database
connections in use and idle against the pool's maximum, commands waiting
for the worker, live-update listeners, free torrent slots, episodes
playing through DeadAir, the library's counts, the tokio runtime's
workers, tasks and queue, and the process's memory and open files, in
Prometheus's text format, written by hand rather than with a client
library. Not logged or traced: Prometheus asks every 15 s. The iPX
dashboard gains a Daemon row of them.

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

src/db.rs

@@ -130,6 +130,24 @@ pub fn location() -> String {
 /// The Postgres connection option that keeps notices from being sent at all; see `connect`.
 const QUIET: &str = "options=-c%20client_min_messages%3Dwarning";
 
+impl Db {
+    /// The connection pool as it stands, for /metrics (#194): connections open, those of them
+    /// idle, and the most it will open.
+    pub fn pool(&self) -> (u32, usize, u32) {
+        use sea_orm::DbBackend;
+        match self.orm.get_database_backend() {
+            DbBackend::Postgres => {
+                let p = self.orm.get_postgres_connection_pool();
+                (p.size(), p.num_idle(), p.options().get_max_connections())
+            }
+            _ => {
+                let p = self.orm.get_sqlite_connection_pool();
+                (p.size(), p.num_idle(), p.options().get_max_connections())
+            }
+        }
+    }
+}
+
 /// A URL fit for a log or an error: the password taken out.
 fn redact(url: &str) -> String {
     match (url.find("://"), url.rfind('@')) {