fix: UTF-8 encoding and DML support for app.py

[Cerco01]

What and why

While completing the SQL analysis exercise I ran into two bugs in app.py that affect all students. I investigated the root cause
of each one and fixed them after finishing the exercise.

Changes in src/app.py

FIX 1 — UTF-8 encoding
The original open() call did not specify an encoding, so Python used the OS default (cp1252 on Windows). This caused the error:
charmap codec can't decode byte 0x8f
because queries.sql contains UTF-8 characters (accents, emojis).
Fix: added encoding='utf-8' explicitly to the open() call.

FIX 2 — DML support (INSERT, UPDATE, DELETE)
pd.read_sql() only handles SELECT statements, as it expects rows in return. For INSERT/UPDATE/DELETE it raised:
This result object does not return rows. It has been closed automatically.
Fix: detect the statement type and route DML operations to engine.begin() with sqlalchemy.text(), which handles transactions
correctly. SELECT statements continue using pd.read_sql().

Note

Both fixes are documented with inline comments in app.py explaining the original issue and the solution applied.