feat(esm_catalog): PR-C1 — middleware + export utilities

[siligam]

Summary

Provenance: this implementation is substantially Paul Gierz's work, carried over from his esm-tools-plus/simcat/pgierz-workbench prototype branch (commits Mar–Apr 2026). This PR ports it, largely as-is, into the reviewable stack.

• Adds api/middleware.py: CorrelationIdMiddleware (sets request.state.correlation_id to 8-char UUID) + RequestLoggingMiddleware (logs incoming method/path and outgoing status/latency; handles X-Forwarded-For)
• Adds storage/export.py: export_parquet() + export_json() for writing STAC items to disk; import_parquet() for the SLURM parallel-scan merge workflow; pyarrow is a lazy import (only needed at call time, not install time)
• Wires RequestLoggingMiddleware into create_app() middleware stack (before CORS)
• 8 new tests (137 total green); 2 pyarrow tests auto-skip when pyarrow isn't installed

Test plan

• pytest src/esm_catalog/tests/test_middleware_export.py -v — 8 pass (2 pyarrow skipped)
• pytest src/esm_catalog/tests/ — 137 total green
• CI esm-catalog-tests passes on 3.9 + 3.12

Stack position

release → PR-0 → A1a → A1b-1 → A1b-2 → A2 → A3 → A4 → B1 → B2 → B3 → B4 → C1 (this PR)

🤖 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.