feat: add Convex project backend with generic storage and org-scoped authz

Schema:
- Cut projects table to {organizationId, name, description, createdAt, updatedAt}
- Add projectSources table with git source metadata and normalized URL index
- Add projectContextDocuments table with six canonical kinds, origin, revision
- Add by_organizationId index for project enumeration
- Remove all GitHub-specific fields (ownerId, provider, repoOwner, etc.)

Backend:
- Add requireCurrentOrganization and requireProjectMember authz helpers
- Migrate signals.ts authorizeProject to organizationId invariant check
- Update artifactModel: 8 operational artifacts (removed project/business/design.md)
- Add projectStore.ts with query/mutation/action store adapters
- Rewrite projects.ts to thin generic application calls
- Update projectIssues/projectArtifacts to use requireProjectMember
- Update projectEvents.test.ts and signals.test.ts for new project creation

All 33 backend tests pass.

packages/backend/convex/projectIssues.ts

@@ -2,20 +2,7 @@ import { ConvexError, v } from "convex/values";
 
 import type { Id } from "./_generated/dataModel";
 import { mutation, query } from "./_generated/server";
-import type { MutationCtx } from "./_generated/server";
-import { requireAuthUserId } from "./authz"
-
-const requireProjectOwner = async (
-  ctx: MutationCtx,
-  projectId: Id<"projects">,
-  ownerId: string
-) => {
-  const project = await ctx.db.get("projects", projectId);
-  if (!project || project.ownerId !== ownerId) {
-    throw new ConvexError("Project not found");
-  }
-  return project;
-};
+import { requireProjectMember } from "./authz";
 
 export const create = mutation({
   args: {
@@ -24,8 +11,7 @@ export const create = mutation({
     title: v.string(),
   },
   handler: async (ctx, args) => {
-    const ownerId = await requireAuthUserId(ctx);
-    await requireProjectOwner(ctx, args.projectId, ownerId);
+    await requireProjectMember(ctx, args.projectId);
     const title = args.title.trim();
     const body = args.body.trim();
     if (title.length < 3 || title.length > 160) {
@@ -81,12 +67,11 @@ export const create = mutation({
 export const begin = mutation({
   args: { issueId: v.id("projectIssues") },
   handler: async (ctx, args) => {
-    const ownerId = await requireAuthUserId(ctx);
     const issue = await ctx.db.get("projectIssues", args.issueId);
     if (!issue) {
       throw new ConvexError("Issue not found");
     }
-    await requireProjectOwner(ctx, issue.projectId, ownerId);
+    await requireProjectMember(ctx, issue.projectId);
     if (issue.status === "queued" || issue.status === "working") {
       return issue.status;
     }
@@ -109,12 +94,11 @@ export const begin = mutation({
 export const markDispatchFailed = mutation({
   args: { error: v.string(), issueId: v.id("projectIssues") },
   handler: async (ctx, args) => {
-    const ownerId = await requireAuthUserId(ctx);
     const issue = await ctx.db.get("projectIssues", args.issueId);
     if (!issue) {
       throw new ConvexError("Issue not found");
     }
-    await requireProjectOwner(ctx, issue.projectId, ownerId);
+    await requireProjectMember(ctx, issue.projectId);
     const timestamp = Date.now();
     await ctx.db.patch("projectIssues", issue._id, {
       status: "failed",
@@ -133,9 +117,9 @@ export const markDispatchFailed = mutation({
 export const list = query({
   args: { projectId: v.id("projects") },
   handler: async (ctx, args) => {
-    const ownerId = await requireAuthUserId(ctx);
-    const project = await ctx.db.get("projects", args.projectId);
-    if (!project || project.ownerId !== ownerId) {
+    try {
+      await requireProjectMember(ctx, args.projectId);
+    } catch {
       return [];
     }
     return await ctx.db