feat: add cleanup strength levels

[adamholter]

What changed

• replaces the cleanup on/off control with None, Light, Medium, and High contracts
• preserves legacy behavior by migrating enabled users to Medium and disabled users to None
• keeps custom prompts authoritative instead of silently appending level instructions
• records the applied level in transcript history with validated database provenance
• updates all current locales and adds focused migration, prompt, persistence, and UI coverage

Why

Users currently choose only whether AI cleanup runs. Explicit levels make the fidelity-versus-polish tradeoff predictable without changing the established default.

Validation

• 268 passing tests, 0 failures, 5 macOS-expected Linux skips
• formatting, TypeScript, i18n, and renderer build pass
• headless settings and history flows passed with zero console errors
• live API-only semantic fixture: None 246 chars, Light 215, Medium 215, High 194; all eight required names, numbers, dates, and technical terms were preserved

The live fixture used synthetic text and an existing server-side provider credential; no user content or secret was included in the artifact.