feat: add project_goals many-to-many join table

Create project_goals join table with composite PK (project_id, goal_id),
backfill from existing projects.goal_id, and update the project service
to read/write through the join table. Shared types now include goalIds
and goals arrays on Project. Legacy goalId column is kept in sync.

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

packages/shared/src/validators/project.ts

@@ -2,7 +2,9 @@ import { z } from "zod";
 import { PROJECT_STATUSES } from "../constants.js";
 
 export const createProjectSchema = z.object({
+  /** @deprecated Use goalIds instead */
   goalId: z.string().uuid().optional().nullable(),
+  goalIds: z.array(z.string().uuid()).optional(),
   name: z.string().min(1),
   description: z.string().optional().nullable(),
   status: z.enum(PROJECT_STATUSES).optional().default("backlog"),