feat(esm_catalog): PR-B3 — experiment hierarchy routes + paleo presets

[siligam]

Summary

Provenance: api/paleo_presets.py is ported from Paul Gierz's esm-tools-plus/simcat/pgierz-workbench prototype branch; api/experiment_routes.py is new for this decomposition.

• api/experiment_routes.py: GET /experiments (list with CQL2 filter support), GET /experiments/{id} (STAC Catalog object), GET /collections/{id}/experiment (shortcut from collection to parent experiment)
• api/paleo_presets.py: in-memory DuckDB table of 9 built-in paleo periods (LGM, Mid-Holocene, Eemian, etc.); POST /paleo-presets to add custom presets, DELETE /paleo-presets/{id} for user-added presets only
• Adapted from source: removed paleodatetime dependency (not in project requirements); fixed DuckDB rowcount bug (always returns -1 on DELETE — now checks existence before delete)
• 12 new tests (113 total); no new dependencies

Test plan

• CI green on 3.9 + 3.12
• GET /experiments → list with numberMatched
• GET /experiments/{id} → {"type": "Catalog", ...}
• GET /collections/{id}/experiment → parent experiment catalog
• GET /paleo-presets → list including lgm
• Add + delete custom preset roundtrip
• Delete built-in preset → 404

🤖 Generated with Claude Code

[siligam]

Rebased onto the corrected PR-0 base (#1483) — test relocation to tests/test_esm_catalog/, restored dedicated CI workflow at the new path, and a couple of CI fixes (doctest-collection exclusion, eval_type_backport for pydantic on Python 3.9). No change to this PR's own diff/scope. CI is green.