chore: drop unused seaborn, add scipy to requirements

[AmirhosseinHonardoust]

Aligns requirements.txt with actual imports.

• Remove seaborn: not imported anywhere in src/, app.py, or tests/.
• Add scipy: imported directly (scipy.sparse) in src/explain.py and
  app.py; previously satisfied only transitively via scikit-learn,
  which is fragile.

Verified locally:

• 34/34 tests pass.
• Full smoke pipeline (generate -> data_prep -> train -> evaluate ->
  score) runs clean with seaborn removed.
• Core deps install and import in a fresh virtualenv.