feat: make pandas an optional extra with proper CI matrix

[JesuFemi-O]

Background

Pandas is currently a core dependency pulled in to support dlt's filesystem source (used by the csv-import template). It was previously a transitive dep of dagster; after dropping dagster in #147 it had to become explicit.

Pandas + numpy is ~35MB of install weight that most users don't need.

Goal

Move pandas to an optional extra (pip install 'database-tycoon[pandas]') without breaking the csv-import template or its tests.

Work required

Before making the change, do a full sweep of the repo to understand every place pandas is needed (directly or transitively via dlt sources), then plan the full strategy:

1. Audit — find every import path that touches pandas (our code + dlt source types used in templates)
2. Lazy imports — any tycoon-owned code that imports pandas should do so lazily with a clear error pointing at [pandas]
3. Test markers — add @pytest.mark.pandas marker; gate csv-import tests behind it
4. CI matrix — two jobs: test-core (no pandas, runs pytest -m "not pandas") and test-pandas (installs [pandas], runs pytest -m pandas); core job asserts pandas is not importable
5. Recipe doctest harness — add requires= attribute to the <!-- tycoon-test --> marker grammar so README blocks can declare optional deps

References

• Dropped in feat: drop FastAPI server, Dagster orchestration, and nao/ask extras #147 (dagster removal surfaced this)
• Pattern: dlt itself uses dlt[pandas] for the same reason