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'

app/models/__init__.py

@@ -9,6 +9,7 @@ from .genetic_algorithm import GeneticAlgorithm, ReservationOptimizer
 from .role import Role, Permission, SYSTEM_MODULES
 from .milestone import MilestoneType, AcademicMilestone, BASE_MILESTONE_TYPES
 from .audit_log import AuditLog
+from .enrollment import StudentEnrollment
 
 __all__ = [
     'User',
@@ -29,5 +30,6 @@ __all__ = [
     'MilestoneType',
     'AcademicMilestone',
     'BASE_MILESTONE_TYPES',
-    'AuditLog'
+    'AuditLog',
+    'StudentEnrollment'
 ]

app/models/enrollment.py

@@ -0,0 +1,49 @@
+from app import db
+from datetime import datetime
+
+
+class StudentEnrollment(db.Model):
+    """Association table: student (User) ↔ Commission enrollment managed by Bedelía."""
+    __tablename__ = 'student_enrollments'
+
+    id = db.Column(db.Integer, primary_key=True)
+    student_id = db.Column(
+        db.Integer,
+        db.ForeignKey('users.id', ondelete='CASCADE'),
+        nullable=False,
+        index=True
+    )
+    commission_id = db.Column(
+        db.Integer,
+        db.ForeignKey('commissions.id', ondelete='CASCADE'),
+        nullable=False,
+        index=True
+    )
+    status = db.Column(db.String(20), nullable=False, default='activo')  # activo | retirado | condicional
+    enrolled_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = db.Column(db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    notes = db.Column(db.Text, nullable=True)
+
+    # Unique constraint: a student can only be enrolled once per commission
+    __table_args__ = (
+        db.UniqueConstraint('student_id', 'commission_id', name='uq_student_commission'),
+    )
+
+    # Relationships
+    student = db.relationship('User', backref=db.backref('enrollments', lazy='dynamic', cascade='all, delete-orphan'))
+    commission = db.relationship('Commission', backref=db.backref('enrollments', lazy='dynamic', cascade='all, delete-orphan'))
+
+    def to_dict(self):
+        return {
+            'id': self.id,
+            'student_id': self.student_id,
+            'student_name': self.student.name if self.student else None,
+            'student_email': self.student.email if self.student else None,
+            'commission_id': self.commission_id,
+            'status': self.status,
+            'enrolled_at': self.enrolled_at.isoformat() if self.enrolled_at else None,
+            'notes': self.notes,
+        }
+
+    def __repr__(self):
+        return f'<StudentEnrollment student={self.student_id} commission={self.commission_id} status={self.status}>'