Consume codegen endpoint registry (pilot: CexCandle)

[martinkersner]

Summary

First step toward single-sourcing endpoint metadata from datamaxi-codegen. Adds the generated registry and a base-class helper, and pilots the pattern on CexCandle. No public interface change.

• datamaxi/_endpoints.py (generated) — ENDPOINTS[op_id] → path, method, tag, requires_auth, group/subgroup, and per-param {required, in, type, default, enum, description}; regenerated from the backend OpenAPI spec via make update-python in datamaxi-codegen.
• API.request_endpoint(op_id, **params) — looks up the registry as the source of truth for the URL path, HTTP method, path/query split, required params, and defaults; interpolates in:path params; sends. Additive — query/send_request untouched.
• CexCandle refactored onto it. Removed: hardcoded url_path strings + manual param-dict assembly. Kept hand-written: signatures, doc-link docstrings, semantic checks (market ∈ [spot, futures], "no data found"), and DataFrame conversion.
• Lint: _endpoints.py is generated, so it's excluded from black (pyproject.toml force-exclude) and flake8 (setup.cfg exclude). Emitter output only needs to be valid Python, not formatter-clean.

Interface impact

None. Method signatures, defaults, return types, wire params, and error behavior are identical. The pre-existing test_cex_candle tests (which assert wire params + errors) pass unmodified — that's the proof the observable contract didn't move. request_endpoint is a new additive method on API.

Tests

• black --check + flake8 on datamaxi/tests: clean (generated file correctly excluded).
• pytest: 113 passed, 127 skipped (skips are the live integration/smoke lanes needing an API key).

Regenerating

_endpoints.py is generated — edit endpoints in the backend spec, then from datamaxi-codegen run make update-python DATAMAXI_PYTHON=/path/to/datamaxi-python (see codegen PR #19). Do not hand-edit.

Follow-up (not in this PR)

• Roll the pattern out to the remaining client classes.
• Decide whether the registry should own required/enum validation too (would trim more hand code but changes some error messages).
• Wire codegen's make check-python drift-guard into CI.