ci: extract reusable checks workflow + add scheduled dependency check

[lesnik512]

Brings that-depends in line with the rest of the org: extracts the CI checks into a reusable workflow and adds the weekly scheduled dependency check (same pattern as modern-di, faststream-outbox, and the 12 repos in the recent rollout).

Changes

• _checks.yml (new) — the existing lint + pytest jobs moved verbatim into a reusable on: workflow_call workflow. Matrix (python 3.10–3.14 × faststream <0.6.0/>=0.6.0) and the codecov upload are unchanged.
• ci.yml — now just the push/pull_request triggers + concurrency, delegating to _checks.yml via uses: with secrets: inherit (so CODECOV_TOKEN still reaches the codecov step).
• scheduled.yml (new) — runs _checks.yml on a Mon 06:00 UTC cron (+ manual workflow_dispatch), also with secrets: inherit. On a scheduled failure, a report-failure job opens/updates a single scheduled-failure tracking issue.
• report-scheduled-failure.sh (new) — idempotent issue reporter; likely-cause hint lists this repo's actual stack (ruff, mypy, pyrefly, pytest, faststream).

Why
Catches dependency drift on a schedule before it surprises a normal CI run.

Note: job/check names change from lint / pytest (…) to checks / lint / checks / pytest (…). main has no required-status-check contexts pinned, so this won't block merges — but update any branch-protection rules that reference the old names if you add them later.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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