Give every API value with a unit as a number (closes #199)

A value with a unit is now a number, and its field name ends in the
unit, as the vendored API_POLICIES.md requires. @meta.execution_time
becomes execution_time_ms. In the stats answers, uptime,
connection_duration, memory_usage and whois_stats.current_interval
become uptime_seconds, connection_duration_seconds, memory_usage_bytes
and whois_stats.current_interval_seconds. connection_duration_seconds
is null while the feed is not connected. The status page formats the
numbers itself. REPO_POLICIES.md is re-vendored from the same
sneak/prompts commit.

Model: opus-5-5

internal/server/middleware.go

@@ -53,6 +53,15 @@ func (rw *responseWriter) Header() http.Header {
 	return rw.ResponseWriter.Header()
 }
 
+// The @meta object JSONResponseMiddleware adds to a JSON answer: its key, and
+// the keys of its fields.
+const (
+	jsonKeyMeta            = "@meta"
+	metaKeyTimeZone        = "time_zone"
+	metaKeyAPIVersion      = "api_version"
+	metaKeyExecutionTimeMs = "execution_time_ms"
+)
+
 // JSONResponseMiddleware is an HTTP middleware that wraps all JSON responses
 // with a @meta field containing execution metadata. The metadata includes the
 // time zone (always UTC), API version, and request execution time in milliseconds.
@@ -106,10 +115,10 @@ func JSONResponseMiddleware(next http.Handler) http.Handler {
 		}
 
 		// Add @meta field
-		originalResponse["@meta"] = map[string]any{
-			"time_zone":      "UTC",
-			"api_version":    1,
-			"execution_time": fmt.Sprintf("%d ms", execTime.Milliseconds()),
+		originalResponse[jsonKeyMeta] = map[string]any{
+			metaKeyTimeZone:        "UTC",
+			metaKeyAPIVersion:      1,
+			metaKeyExecutionTimeMs: execTime.Milliseconds(),
 		}
 
 		// Write the enhanced response