Add edit RSVP modal, plus-one tracking, and unified signup form

Merge RSVP + slot claim into a single form. Add plus_one field to RSVPs.
Add clickable RSVP names that open a modal to edit name/note/plus_one.

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

db/models.go

@@ -35,6 +35,7 @@ type Rsvp struct {
 	EventID   int64
 	Name      string
 	Note      string
+	PlusOne   int64
 	CreatedAt time.Time
 }