v1.3.0

Add traffic-source segmentation to results (v1.3.0)

Capture first-touch traffic source (channel, source/medium, campaign,
referrer host, device) on each new sticky assignment, then break down
experiment results by it on the dashboard so a variant's hidden win or
loss inside one audience becomes visible.

Attribution lives at the assignment grain: source is detected once, when
the sticky row is created (before any redirect variant fires), and every
later event the visitor records inherits it via a join, so an off-target
thank-you-page conversion still attributes to the channel the visitor
first arrived through.

- New PBD_Exp_Traffic_Source classifier (GA-style, filterable rules)
- Six source columns on the assignments table, DB_VERSION 1.3.0
- Allowlist-guarded grouped repo queries for visitors/conversions
- Dashboard "Break down by traffic source" card with Channel / Source /
  Medium / Device tabs, low-sample dimming, and a diagnostic caution
- Channel + Device breakdowns frozen into the conclusion snapshot;
  archive renders them read-only and stays backward-compatible with
  pre-feature snapshots

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>