Make dependency audits weekly and tiered

[ArturSepp]

Daily all-extras audits made an optional dependency advisory fail the whole repository repeatedly. This change standardizes the package security policy:

• run the scheduled audit weekly on Monday;
• keep latest and locked core dependency audits blocking;
• audit public optional extras separately as advisory checks;
• enable grouped weekly Dependabot updates for uv and GitHub Actions.

StochVolModels additionally updates jupyter-server from 2.20.0 to 2.21.1, resolving CVE-2026-86049. No package version or release is required.

Validation: all workflow and Dependabot YAML parsed; every pyproject.toml parsed; all ten repositories pass uv lock --check with their external Python 3.12 interpreter.