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/lib/projects.ts

@@ -0,0 +1,43 @@
+import { and, eq } from "drizzle-orm";
+import { db } from "@/lib/db/client";
+import { projects } from "@/lib/db/schema";
+
+/**
+ * Look up a project id by (user, key). Returns null when not found.
+ * No write side-effects.
+ */
+export async function resolveProjectId(
+  userId: string,
+  key: string,
+): Promise<string | null> {
+  const row = await db
+    .select({ id: projects.id })
+    .from(projects)
+    .where(and(eq(projects.userId, userId), eq(projects.key, key)))
+    .limit(1);
+  return row[0]?.id ?? null;
+}
+
+/**
+ * Idempotent project creation. Returns the existing row's id when one
+ * exists, otherwise inserts and returns the new id. Tolerates concurrent
+ * inserts via ON CONFLICT — two simultaneous calls converge on one row.
+ */
+export async function upsertProject(
+  userId: string,
+  key: string,
+  displayName?: string,
+): Promise<string> {
+  const existing = await resolveProjectId(userId, key);
+  if (existing) return existing;
+  const row = await db
+    .insert(projects)
+    .values({ userId, key, displayName: displayName ?? null })
+    .onConflictDoNothing({ target: [projects.userId, projects.key] })
+    .returning({ id: projects.id });
+  if (row[0]) return row[0].id;
+  // ON CONFLICT DO NOTHING returns no rows on conflict — re-read.
+  const reread = await resolveProjectId(userId, key);
+  if (!reread) throw new Error("project upsert raced and re-read still empty");
+  return reread;
+}