Add global species coverage against eBird taxonomy

[jimchurches]

Summary

Adds world species coverage metrics so birders can see how their personal species list compares to the current eBird/Clements checklist. Metrics are computed once during rankings/families bundle prep and shared by both UI surfaces.

Changes

• Taxonomy bundle: parse and retain EXTINCT, EXTINCT_YEAR, and is_extinct from the eBird taxonomy CSV
• Configuration: TAXONOMY_INCLUDE_EXTINCT_SPECIES_IN_COVERAGE = False in defaults.py (extinct species excluded from coverage by default)
• Bird Families overview: fold world species rows into existing sections — Total species under Taxonomy; Observed species and Observed species (%) under Coverage
• Ranking & Lists → Interesting Lists: new Species: Coverage expander (first on the tab) with taxonomy total, observed count, percentage, and extinct-species footnote
• Performance: store world_species_coverage_metrics on the shared bundle; Bird Families and Interesting Lists reuse the same precomputed values

Testing

• python3 -m ruff check explorer/
• python3 -m pytest tests/ -q — 631 passed, 7 skipped
• Manual: load Bird Families overview (no family selected) and Interesting Lists → Species: Coverage

Notes

• User observations roll up to countable base species; unmatched taxonomy rows are excluded from the numerator (debug-logged only)
• Footnote wording reflects TAXONOMY_INCLUDE_EXTINCT_SPECIES_IN_COVERAGE (excluded by default; switches to “included” if the flag is enabled)

Refs #262

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