feat: add audit logging and lesson statistics for clazz

- Add clazz_audit_log table with diesel migration for CRUD audit trail
- Add audit log backend: model, queries, handler, route
- Add audit log viewer in clazz detail modal (操作记录)
- Add student_lesson_stats API (GET /api/v1/clazz/stats/my-lessons)
- Add teacher_detail_stats API (GET /api/v1/clazz/stats/teacher-detail)
- Register all new routes in lib.rs

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

htykc_models/migrations/2026-05-01-113450_create_clazz_audit_log/down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS clazz_audit_log;

htykc_models/migrations/2026-05-01-113450_create_clazz_audit_log/up.sql

@@ -0,0 +1,13 @@
+CREATE TABLE clazz_audit_log (
+    id VARCHAR PRIMARY KEY,
+    clazz_id VARCHAR NOT NULL,
+    action VARCHAR NOT NULL,
+    operator_hty_id VARCHAR NOT NULL,
+    operator_name VARCHAR,
+    changes JSONB,
+    created_at TIMESTAMP NOT NULL DEFAULT NOW(),
+    org_id VARCHAR
+);
+
+CREATE INDEX idx_clazz_audit_log_clazz_id ON clazz_audit_log (clazz_id);
+CREATE INDEX idx_clazz_audit_log_created_at ON clazz_audit_log (created_at);