feat(phase-4a): groups sync + X-Project-Key header substrate

Foundational work for the upcoming group-scoped sharing feature.

Schema (migration 0003_groups.sql + drizzle schema):
  - memory_access enum ('ro' | 'rw') reserved for Agent B's project_shares
  - groups (id, oidc_iss, name, display_name, …) keyed by (oidc_iss, name)
    so different IdPs can both have e.g. "platform" without colliding
  - user_groups (user_id, group_id, synced_at) PK (user_id, group_id)

Auth (auth.ts + lib/auth/sync-groups.ts):
  - jwt callback now syncs `profile.groups` after upserting the user
  - syncUserGroupsFromClaim runs in a single tx: upserts each group,
    inserts new memberships, deletes ones no longer in the claim
  - missing/empty claim → user has zero groups (wipe memberships)
  - EntraID GUID-vs-name edge case: we treat whatever strings the claim
    emits as names verbatim; groups overage (>200 groups → no claim)
    is documented as unsupported in v1

UserContext + JWT (lib/mcp/context.ts, lib/auth/jwt.ts):
  - AuthenticatedClaims.groups surfaced from verified JWT payload
  - UserContext.groups: string[] — live from OIDC token claim, falls
    back to DB snapshot for CLI (HMAC) tokens which carry no claim
  - UserContext.defaultProjectKey: optional, set from header

MCP route (app/api/mcp/route.ts):
  - reads X-Project-Key header, validates against ProjectKey Zod schema,
    400 on invalid; empty/missing leaves defaultProjectKey undefined
  - auto-upserts the header-supplied project so first-use works without
    a separate project.identify call

Tools (lib/mcp/tools.ts):
  - withDefaultProject helper injects ctx.defaultProjectKey when the
    caller omits `project`. Per-tool defaultScope hint avoids breaking
    snippet.put (user-scope default) while making memory.write
    (project-scope default) honor the header
  - applied to memory.write/list/search/update and all snippet.* tools

Web UI:
  - /settings/groups debug page lists current memberships with synced_at
    and a clear empty state pointing at README troubleshooting
  - /settings/tokens grows a "Pin to project" dropdown; selected key is
    baked into the generated `claude mcp add` snippet as
    `--header "X-Project-Key: <key>"`. The JWT itself stays
    identity-only — pinning is purely a UX shortcut
  - settings landing page links to /settings/groups
  - README troubleshooting bullet covers the empty-groups path for
    Authentik / EntraID / Keycloak

Refactor:
  - extracted resolveProjectId + upsertProject from memory-actions.ts
    into lib/projects.ts so the MCP route can reuse upsertProject

Verification:
  - pnpm typecheck clean
  - SKIP_ENV_VALIDATION=true pnpm build clean; /settings/groups in route table

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/auth.ts

@@ -2,6 +2,7 @@ import NextAuth from "next-auth";
 import { env } from "@/lib/env";
 import { db } from "@/lib/db/client";
 import { users } from "@/lib/db/schema";
+import { syncUserGroupsFromClaim } from "@/lib/auth/sync-groups";
 
 /**
  * NextAuth (Auth.js v5) configuration.
@@ -60,9 +61,21 @@ export const { auth, handlers, signIn, signOut } = NextAuth({
           })
           .returning({ id: users.id });
 
-        token.userId = row[0]?.id;
+        const userId = row[0]?.id;
+        token.userId = userId;
         token.sub = sub;
         token.iss = iss;
+
+        // Sync group memberships from the OIDC `groups` claim. Missing or
+        // empty claim is treated as "user is in zero groups" — that path
+        // wipes the user's existing memberships, which is the conservative
+        // choice (don't keep stale grants alive if the IdP stopped
+        // asserting them).
+        if (userId) {
+          // `profile.groups` is untyped at the next-auth boundary — coerce.
+          const claimGroups = (profile as { groups?: unknown }).groups;
+          await syncUserGroupsFromClaim(userId, iss, claimGroups);
+        }
       }
       return token;
     },