Calendar engine: layered observances + brand safety

The content calendar now blends strategy-led generation with a layered observance system (holidays, awareness days, industry events) and a brand-safety filter that runs before posts are surfaced. Output is materially more on-brand and less generic.

Pattern library is modular — designers and ops can add new content patterns without touching the generator core. Continuation context lets the AI build on prior calendars instead of starting fresh each time.