Function effective_timezone 

pub fn effective_timezone(
    own: Option<&str>,
    policy_repo_defaults: &[Option<String>],
) -> (Tz, Option<TimezoneAmbiguity>)

Pure. Decide the effective timezone a SnapshotSchedule’s cron is evaluated in, given the schedule’s own spec.schedule.timezone (own) and the scheduleDefaults.timezone of each matched target policy’s repository (policy_repo_defaults, one entry per matched policy; None = that repo sets no default). Mirrors resolve_tz fallback semantics (an unparseable name degrades to UTC — the webhook rejects bad names up front).

Rules (the reconciler does the GETs and passes the data in):

• own set → that zone wins, no lookups, never ambiguous.
• own unset, no matched policies → UTC, not ambiguous.
• own unset, all matched policies resolve to one zone → that zone.
• own unset, matched policies resolve to differing zones → UTC plus a TimezoneAmbiguity (recommend an explicit spec.schedule.timezone).

A single policyRef therefore never yields ambiguity (one repository). Repos with no default resolve to UTC, so mixing “a zone” with “no default” is a genuine disagreement and is reported.

use kopiur_api::common::effective_timezone;

// Own timezone wins outright.
let (tz, amb) = effective_timezone(Some("America/Chicago"), &[]);
assert_eq!(tz.name(), "America/Chicago");
assert!(amb.is_none());

// Unset own, one agreeing default across matched policies.
let defs = [Some("Europe/Berlin".to_string()), Some("Europe/Berlin".to_string())];
let (tz, amb) = effective_timezone(None, &defs);
assert_eq!(tz.name(), "Europe/Berlin");
assert!(amb.is_none());

// Unset own, disagreeing defaults → UTC + ambiguity signal.
let defs = [Some("Europe/Berlin".to_string()), None];
let (tz, amb) = effective_timezone(None, &defs);
assert_eq!(tz, chrono_tz::Tz::UTC);
assert!(amb.is_some());