Replace the custom now() function with an idiomatic `now` timestamp variable (host-injected, frozen once per compile) and retype created_ts/updated_ts/create_time to CEL timestamp. Filters now use standard timestamp/duration arithmetic, e.g. `created_ts >= now - duration("24h")` and `timestamp("2025-01-01T00:00:00Z")`.
Add standard CEL surface that compiles to SQL across SQLite/MySQL/Postgres: timestamp accessors (getFullYear/getMonth/getDate/getDayOfWeek/..., with 0-based month and weekday normalized), ext.Sets() (sets.contains/intersects/equivalent over tags), tags.exists_one(), size() on string fields, and division/modulo folding. A frozen clock is injectable for deterministic tests.
BREAKING CHANGE: now() is removed (use the `now` variable) and time fields are timestamps, so bare-epoch comparisons need timestamp(<epoch>). Existing saved shortcuts using the old syntax must be updated.
