feat(badges): configurable header image vertical offset per template

Adds cfg_json.header_margin_top to BadgeTemplateCfg. Badge view replaces
hardcoded mt-8 (2rem) with this value; falls back to 2rem when unset.
Template form exposes the field in the Header & Branding section.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/ae_events/types/ae_badge_template_cfg.ts

@@ -36,6 +36,13 @@ export interface BadgeTemplateCfg {
     // Leave unset (or "0") for no bleed.
     bleed?: string;
 
+    // Header image vertical offset. CSS length applied as margin-top on the badge_header div.
+    // Default (unset) = "2rem" (matches the prior hardcoded mt-8).
+    // Negative values shift the image toward the top edge; larger values push it down.
+    // Useful when a background image's designed zone doesn't align with the default position.
+    // Any CSS length works: "-0.5in", "1rem", "8px".
+    header_margin_top?: string;
+
     // Allow arbitrary extra keys to preserve forward-compatibility.
     [key: string]: any;
 }