Phase 3: individual defense assignments with invite codes

- progress V5 defense_sessions: per student+lab individual task (md), status
  machine waiting -> started -> ready, one-time 6-digit codes
- 30-min timer with server-side lazy expiry (syncTimeouts: started->ready by
  timeout), countdown JS on student page
- admin: assign task dialog (+prefill existing via data-md), CSV import
  (studentNo;task, \n for newlines) resolved via identity students,
  issue/reset code buttons, defense status column
- student: code entry form (code hidden until start), assignment md,
  'Готов к сдаче', waiting-for-teacher note
- gateway: /api/v1/defenses,/api/v1/defenses/** routes
- web api client + admin/student controllers wired

services/progress-service/src/main/resources/db/migration/V5__defense_sessions.sql

@@ -0,0 +1,16 @@
+-- Defense sessions: individual task + invite code + 30-minute timer.
+
+CREATE TABLE defense_sessions (
+    id            uuid PRIMARY KEY DEFAULT uuidv7(),
+    student_id    uuid    NOT NULL,
+    lab_id        uuid    NOT NULL,
+    assignment_md text    NOT NULL DEFAULT '',
+    status        varchar(16) NOT NULL DEFAULT 'waiting',  -- waiting | started | ready
+    code          varchar(6),
+    started_at    timestamptz,
+    ready_at      timestamptz,
+    ready_reason  varchar(16),                              -- button | timeout
+    CONSTRAINT defense_session_unique UNIQUE (student_id, lab_id)
+);
+
+CREATE INDEX defense_sessions_lab_idx ON defense_sessions (lab_id);