fix: remove dead migration SQL that references nonexistent tables (issue #1877)

[ionfwsrijan]

Problem

The migrations/ directory is disconnected from the application's real schema. Its SQL files reference tables (paths, certificates, discussion_threads, thread_comments, bookmarks, user_topic_progress, user_activity) that no SQLAlchemy model defines, and 001/002 declare FOREIGN KEY ... REFERENCES paths(id) against a paths table that does not exist anywhere — learning paths are stored in-memory. There is also no migration runner (no Alembic/Flask-Migrate config, no code reads the .sql files), so none of the files can be applied in any supported workflow. The app creates its schema with db.create_all() from src/models.py (src/app.py, src/seed_db.py).

Fix

• Removed the five unapplied, schema-drifted SQL files from migrations/.
• Documented the intended data model in migrations/README.md: the schema is defined exclusively by src/models.py (projects, users, project_progress, user_game_progress) and created via db.create_all(); no paths table exists.
• Added tests/test_db_schema_consistency.py — a pure-file CI check that (1) every db.ForeignKey(...) in src/models.py targets a defined table, and (2) any SQL migration in migrations/ may only reference model tables or tables created by an earlier migration in the set, so a dangling FK like REFERENCES paths(id) can never be reintroduced.

Files changed

• migrations/001_create_certificates_table.sql — deleted
• migrations/002_create_discussion_tables.sql — deleted
• migrations/003_create_bookmarks_table.sql — deleted
• migrations/004_create_progress_tables.sql — deleted
• migrations/005_create_user_game_progress.sql — deleted
• migrations/README.md — documents the model-driven schema
• tests/test_db_schema_consistency.py — new consistency/regression check

Testing

• tests/test_db_schema_consistency.py — 2 passed (run with --noconftest since the app can't boot locally due to the pre-existing App fails to boot: NameError: name 're' is not defined in portfolio_analyzer.py #1810).

Closes #1877

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