fix(cli): coerce plan --min-intervals to int

[jthurlburt]

Description

Why. plan --min-intervals is a click option with no type, so the raw string is passed through Context.plan() to range(min_intervals) in _calculate_start_override_per_model, raising TypeError: 'str' object cannot be interpreted as an integer. Any CLI plan that passes --min-intervals and produces backfill work crashes.

What. Add type=int to the option. Valid values backfill as intended; a non-integer now produces a standard click usage error (Error: Invalid value for '--min-intervals': 'abc' is not a valid integer.) instead of a traceback. One-line fix plus a regression test.

Test Plan

• New tests/cli/test_cli.py::test_plan_min_intervals — builds prod, edits a model to force a dev backfill, then asserts plan dev --min-intervals 1 exits 0 and plan dev --min-intervals abc exits 2 with the click error. Verified it fails without the fix (TypeError) and passes with it by reverting type=int.
• Reproduced the original crash on a clean checkout (duckdb example) before applying the fix.
• make style: ruff, ruff-format, mypy, valid-migrations all pass (prettier/eslint skipped — UI only, unrelated).

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test) — see Test Plan note
• My commits are signed off (git commit -s) per the DCO