feat: add share link service, API routes, and short URL redirect

Create share.service.ts with token generation (128-bit base64url),
CRUD operations, validation, and visibility transition side effects.
Add share endpoints under /api/setups/:id/shares, shared access at
/api/shared/:token, and /s/:token short URL redirect.

Plan: 32-02 (Setup Sharing System - Share Link Backend)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/schemas.ts

@@ -96,6 +96,12 @@ export const updateSetupSchema = z.object({
 	visibility: z.enum(["private", "link", "public"]).optional(),
 });
 
+export const createShareLinkSchema = z.object({
+	expiresInDays: z
+		.union([z.literal(7), z.literal(14), z.literal(30), z.null()])
+		.default(14),
+});
+
 export const syncSetupItemsSchema = z.object({
 	itemIds: z.array(z.number().int().positive()),
 });