Add React frontend and Sinbad2IA LLM integration.

Introduce a full Vite/React UI for exams, auth, materials, images, generation, and export.
Adapt backend for Sinbad2IA chat API, bcrypt passwords, CORS on port 5173, and schema migrations.

backend/app/db/init_db.py

@@ -1,7 +1,9 @@
 from app.db.base import Base
+from app.db.migrations import run_migrations
 from app.db.session import engine
 from app.models import exam, user  # noqa: F401
 
 
 def init_db() -> None:
     Base.metadata.create_all(bind=engine)
+    run_migrations(engine)

backend/app/db/migrations.py

@@ -0,0 +1,104 @@
+"""
+Migraciones ligeras e idempotentes para bases de datos creadas antes de nuevas columnas.
+Se ejecutan en cada arranque; solo aplican cambios que falten.
+"""
+
+from sqlalchemy import inspect, text
+from sqlalchemy.engine import Engine
+
+
+def run_migrations(engine: Engine) -> None:
+    inspector = inspect(engine)
+    table_names = set(inspector.get_table_names())
+
+    with engine.begin() as conn:
+        if "users" in table_names and "exam_templates" in table_names:
+            _ensure_exam_templates_user_id(conn, inspector)
+
+        if "questions" in table_names:
+            _ensure_questions_image_id(conn, inspector)
+
+        if "exam_materials" in table_names:
+            _ensure_material_status_enum(conn)
+
+
+def _column_names(inspector, table: str) -> set[str]:
+    return {col["name"] for col in inspector.get_columns(table)}
+
+
+def _ensure_exam_templates_user_id(conn, inspector) -> None:
+    if "user_id" in _column_names(inspector, "exam_templates"):
+        return
+
+    conn.execute(
+        text("ALTER TABLE exam_templates ADD COLUMN user_id UUID")
+    )
+
+    # Plantillas antiguas (sin usuario): asignar al primer usuario registrado.
+    conn.execute(
+        text(
+            """
+            UPDATE exam_templates
+            SET user_id = (SELECT id FROM users ORDER BY created_at ASC LIMIT 1)
+            WHERE user_id IS NULL
+              AND EXISTS (SELECT 1 FROM users)
+            """
+        )
+    )
+    # Si no hay usuarios o filas huérfanas, eliminar plantillas sin dueño.
+    conn.execute(text("DELETE FROM exam_templates WHERE user_id IS NULL"))
+
+    conn.execute(text("ALTER TABLE exam_templates ALTER COLUMN user_id SET NOT NULL"))
+    conn.execute(
+        text(
+            """
+            ALTER TABLE exam_templates
+            ADD CONSTRAINT fk_exam_templates_user_id
+            FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
+            """
+        )
+    )
+    conn.execute(
+        text(
+            "CREATE INDEX IF NOT EXISTS ix_exam_templates_user_id ON exam_templates (user_id)"
+        )
+    )
+
+
+def _ensure_questions_image_id(conn, inspector) -> None:
+    if "image_id" in _column_names(inspector, "questions"):
+        return
+
+    if "exam_images" not in set(inspector.get_table_names()):
+        return
+
+    conn.execute(text("ALTER TABLE questions ADD COLUMN image_id UUID"))
+    conn.execute(
+        text(
+            """
+            ALTER TABLE questions
+            ADD CONSTRAINT fk_questions_image_id
+            FOREIGN KEY (image_id) REFERENCES exam_images(id) ON DELETE SET NULL
+            """
+        )
+    )
+    conn.execute(
+        text("CREATE INDEX IF NOT EXISTS ix_questions_image_id ON questions (image_id)")
+    )
+
+
+def _ensure_material_status_enum(conn) -> None:
+    # Asegura el tipo enum de PostgreSQL usado por exam_materials.status.
+    conn.execute(
+        text(
+            """
+            DO $$
+            BEGIN
+                IF NOT EXISTS (SELECT 1 FROM pg_type WHERE typname = 'materialstatus') THEN
+                    CREATE TYPE materialstatus AS ENUM ('processed', 'failed');
+                END IF;
+            END
+            $$;
+            """
+        )
+    )