feat(server): slog request log + log dropped Lidarr ping errors

Two debuggability gaps surfaced by the /api/admin route shadowing
investigation:

1. handleTestLidarrConnection swallowed client.Ping's error — only the
   bucket code (lidarr_unreachable) reached the response, the underlying
   *net.DNSError / *net.OpError / TLS error never reached the logs.
   Operators couldn't tell a typo'd hostname from a wrong port from a
   refused TLS handshake. Now logged at Warn (expected-when-misconfigured)
   with the base_url for diagnostic context. The api_key is never logged.

2. The chi router had no access-log middleware — every 4xx/5xx was silent,
   making it impossible to tell whether a request even reached the server.
   chi's middleware.Logger writes to the standard log package not slog,
   so a small slog wrapper handles it instead. /healthz is skipped (the
   compose healthcheck hits it every 5s; would be ~17k log lines/day of
   noise). Severity is keyed off response status: 5xx -> Error,
   4xx -> Warn, else Info — so 4xx/5xx surface even when the operator's
   logger level is set above Info.

Tests cover both the severity routing and the /healthz skip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/admin_lidarr.go

@@ -147,6 +147,14 @@ func (h *handlers) handleTestLidarrConnection(w http.ResponseWriter, r *http.Req
 	result, err := client.Ping(r.Context())
 	if err != nil {
 		errCode := lidarrErrCode(err)
+		// Warn (not Error): a failed connection test is expected when the
+		// admin is configuring the integration and may have a typo'd URL
+		// or wrong key. Without this log line the underlying *net.DNSError
+		// / *net.OpError / TLS error is invisible to operators — they only
+		// see the bucket code in the UI. base_url is safe to log; api_key
+		// must NEVER be logged.
+		h.logger.Warn("admin: test lidarr ping failed",
+			"err", err, "base_url", baseURL, "code", errCode)
 		writeJSON(w, http.StatusOK, map[string]any{"ok": false, "error": errCode})
 		return
 	}