refactor: select in tag_service

[RenzoMXD]

Summary

• Migrate all 9 db.session.query() calls to SQLAlchemy 2.x select() style in tag_service.py
• Replace .first() with db.session.scalar() + .limit(1) for single-row queries
• Replace .count() with db.session.scalar(select(func.count(...)))
• Replace .query().join().all() with db.session.scalars(select().join()).all()
• Replace .query().outerjoin().group_by().all() with db.session.execute(select().outerjoin().group_by()).all()
• No test changes needed

Test plan

• All 10 unit tests pass for tag controller tests
• Type checks pass (basedpyright + pyrefly + mypy)

Part of #22668

[github-actions]

Pyrefly Diff

No changes detected.

[github-actions]

Pyrefly Diff

No changes detected.

[Copilot]

Pull request overview

Refactors TagService’s ORM access to SQLAlchemy 2.x select()-style statements to improve typing and align with the project’s ongoing migration away from db.session.query() (per #22668).

Changes:

• Migrated legacy db.session.query(...) usages in TagService to select(...) statements.
• Replaced .first() / .count() patterns with db.session.scalar(...) and select(func.count(...)).
• Updated join-based queries to use db.session.scalars(...) / db.session.execute(...) in 2.x style.

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[Copilot]

db.session.execute(...).all() already returns a list, so wrapping it in list(...) is redundant. Dropping the extra list() avoids an unnecessary copy and keeps the intent clearer.

Suggested change

	results: list = list(db.session.execute(stmt.order_by(Tag.created_at.desc())).all())
	results: list = db.session.execute(stmt.order_by(Tag.created_at.desc())).all()

[RenzoMXD]

Thanks.