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>

migrate.go

@@ -20,6 +20,8 @@ func runMigrations(database *sql.DB) {
 		`CREATE INDEX IF NOT EXISTS idx_verification_codes_identifier ON verification_codes(identifier)`,
 		// Dedupe RSVPs: keep earliest per (event_id, name).
 		`DELETE FROM rsvps WHERE id NOT IN (SELECT MIN(id) FROM rsvps GROUP BY event_id, name COLLATE NOCASE)`,
+		// Plus-one tracking for RSVPs.
+		`ALTER TABLE rsvps ADD COLUMN plus_one INTEGER NOT NULL DEFAULT 0`,
 	}
 	for _, m := range migrations {
 		_, err := database.Exec(m)