Add tidy DataFrame output

[wilsonfreitas]

Summary

• Add tidy=True DataFrame output for SGS and currency sync/async getters
• Keep the existing wide DataFrame output as the default
• Leave output="text" unchanged
• Document tidy schemas: SGS Date/series/value and currency Date/symbol/side/value

Closes #1

Tests

• uv run ruff format --check bcb tests
• uv run ruff check bcb tests
• uv run mypy bcb
• uv run pytest
• uv run --group docs sphinx-build -b html docs docs/_build/html
• git -c core.whitespace=blank-at-eol,blank-at-eof,space-before-tab,cr-at-eol diff --check