Add mid-session quota enforcement with 6 tests

New enforcer.rs module runs alongside active tests:
- Periodic quota checks (default every 10s, configurable --quota-check-interval)
- Max duration enforcement — forcefully stops test after limit
- User quotas: daily/weekly/monthly checked against DB + current session
- IP quotas: daily/weekly/monthly checked against DB + current session
- Flush session bytes to DB for accurate cross-session tracking
- Sets state.running=false to gracefully terminate on quota breach

StopReason enum tracks why a test was stopped:
  MaxDuration, UserDailyQuota, UserWeeklyQuota, UserMonthlyQuota,
  IpDailyQuota, IpWeeklyQuota, IpMonthlyQuota, ClientDisconnected

Tests (6 new, all passing):
- test_enforcer_max_duration: stops after max_duration seconds
- test_enforcer_client_disconnect: detects normal client exit
- test_enforcer_user_daily_quota_exceeded: stops when user quota hit
- test_enforcer_ip_daily_quota_exceeded: stops when IP quota hit
- test_enforcer_under_quota_runs_normally: doesn't stop if under limits
- test_enforcer_flush_records_usage: verifies DB persistence

64 total tests (58 standard + 6 enforcer), all passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server_pro/main.rs

@@ -10,6 +10,7 @@
 
 mod user_db;
 mod quota;
+mod enforcer;
 mod ldap_auth;
 
 use clap::Parser;
@@ -86,6 +87,10 @@ struct Cli {
     #[arg(long = "max-duration", default_value_t = 300)]
     max_duration: u64,
 
+    /// How often to check quotas during a test in seconds
+    #[arg(long = "quota-check-interval", default_value_t = 10)]
+    quota_check_interval: u64,
+
     /// Use EC-SRP5 authentication
     #[arg(long = "ecsrp5")]
     ecsrp5: bool,