feat: gestion de materias por rol - Bedelia, Docente y Alumno

- Nuevo modelo StudentEnrollment (tabla student_enrollments)
  con inscripcion unica por alumno/comision, estado y notas

- Migracion automatica en init_db.py (CREATE TABLE IF NOT EXISTS)

- Rutas /admin/commissions/ para Bedelia y Admin:
  listado con filtros, detalle, asignar docente,
  inscribir/dar de baja alumnos, cambiar estado

- Blueprint my_subjects_bp en /mis-materias:
  Docente ve solo sus comisiones asignadas
  Alumno ve solo sus inscripciones activas
  Admin/Bedelia ven todo

- Templates: admin/commissions/list.html, detail.html
            my_subjects/list.html, detail.html

- Navbar actualizado por rol:
  Docente y Alumno: 'Mis Materias'
  Bedelia y Admin: 'Gestion de Comisiones'

init_db.py

@@ -74,6 +74,20 @@ def init_database():
             db.session.execute(text("""
                 ALTER TABLE reservations ADD COLUMN IF NOT EXISTS shift VARCHAR(30);
             """))
+            db.session.execute(text("""
+                CREATE TABLE IF NOT EXISTS student_enrollments (
+                    id SERIAL PRIMARY KEY,
+                    student_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+                    commission_id INTEGER NOT NULL REFERENCES commissions(id) ON DELETE CASCADE,
+                    status VARCHAR(20) NOT NULL DEFAULT 'activo',
+                    enrolled_at TIMESTAMP WITHOUT TIME ZONE DEFAULT NOW() NOT NULL,
+                    updated_at TIMESTAMP WITHOUT TIME ZONE DEFAULT NOW(),
+                    notes TEXT,
+                    CONSTRAINT uq_student_commission UNIQUE (student_id, commission_id)
+                );
+                CREATE INDEX IF NOT EXISTS ix_student_enrollments_student_id ON student_enrollments (student_id);
+                CREATE INDEX IF NOT EXISTS ix_student_enrollments_commission_id ON student_enrollments (commission_id);
+            """))
             db.session.commit()
             print("[OK] Esquema y tablas actualizadas con éxito.")