Reject session lifetimes finer than stored millisecond precision

MEALQUEST_SESSION_IDLE and MEALQUEST_SESSION_MAX must now be whole
milliseconds: session expiry is stored as a store.Timestamp, so a value
such as 1500us would be truncated and a session could expire early.

Fixes review finding MQ-19-1.

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

internal/config/config_test.go

@@ -269,6 +269,20 @@ func TestLoad(t *testing.T) {
 			env:     with(map[string]string{EnvSessionMax: "-1h"}),
 			badVars: []string{EnvSessionMax},
 		},
+		{
+			name: "whole-millisecond session lifetimes",
+			env:  with(map[string]string{EnvSessionIdle: "1ms", EnvSessionMax: "2ms"}),
+		},
+		{
+			name:    "sub-millisecond session idle",
+			env:     with(map[string]string{EnvSessionIdle: "1500us", EnvSessionMax: "2ms"}),
+			badVars: []string{EnvSessionIdle},
+		},
+		{
+			name:    "sub-millisecond session max",
+			env:     with(map[string]string{EnvSessionIdle: "1ms", EnvSessionMax: "1ms500us"}),
+			badVars: []string{EnvSessionMax},
+		},
 		{
 			name:    "session max below idle",
 			env:     with(map[string]string{EnvSessionIdle: "48h", EnvSessionMax: "24h"}),