Finland

[thiagovmdon]

Title

Related to issue #65

Body

Summary

• Add a new Finland fetcher backed by SYKE Hydrologiarajapinta.
• Support discharge_daily_mean, stage_daily_mean, and water-temperature_daily_mean.
• Use SYKE Paikka_Id as gauge_id, since Nro is not unique across variables.
• Add cached Finland metadata for the supported station set.
• Add raw-payload unit tests and a small example fetcher script.
• Align the Sphinx API page with the Germany Berlin structure:
  Data source, Supported variables, Data description and API, Terms of use, plus the method docs.
• Fix long-range downloads by splitting requests into 365-day windows.

Why the chunking fix is needed

SYKE truncates filtered OData time-series responses at 500 rows and does not return a usable odata.nextLink for these requests. Without chunking, multi-year downloads stop early. This PR avoids that by requesting long date ranges in smaller windows and concatenating the results.

Verification

• ./.venv/bin/python -m unittest discover -s tests -p 'test*.py'
• ./.venv/bin/ruff check .
• ./.venv/bin/ruff format --check .
• ./.venv/bin/sphinx-build -M html docs docs/_build -a -E

Live checks

• Discharge: gauge 897, 2025-01-01 to 2026-03-27 returns 451 rows after the chunking fix.
• Stage: gauge 1900, 2025-01-01 to 2026-03-27 returns 450 available rows.
• Temperature: gauge 3094, 2025-01-01 to 2026-01-01 returns 260 available rows.