fix(audit-wave-11): auth-flow hardening (auth-flow-auditor)

Address the two CRITICAL items from auth-flow-auditor plus the
high-impact M10 open-redirect.

**C1 — Password reset doesn't revoke existing sessions**

CRM side: Better Auth has a built-in
`emailAndPassword.revokeSessionsOnPasswordReset` flag — flip it on.
Verified by reading password.mjs in node_modules/better-auth: this
calls `internalAdapter.deleteSessions(userId)` after the password
update commits. One-line fix, closes the canonical session-bumping
gap on the CRM forgot-password flow.

Portal side: the portal uses JWT sessions (not DB-side rows) so
there's no `deleteSessions` to call. Add a per-user
`password_changed_at` watermark column on `portal_users` and have
`verifyPortalToken` reject any token whose `iat` predates the
watermark. Updated on `resetPassword`, `changePortalPassword`, and
`activateAccount` so every password mutation revokes outstanding
cookies. Token shape gains a required `portalUserId` claim so the
verify step can do the watermark lookup without an email-based join;
legacy tokens (pre-Wave-11) lack it and are rejected → forces one
re-login per portal user post-deploy (24h max delay since portal
tokens already self-expire at 24h).

Migration `0058_portal_password_revocation.sql` stamps existing
rows to `now()` so no current session is invalidated by the schema
change itself.

**M10 — Portal login `?next=` open redirect**

`portal/login/page.tsx` did `router.replace(next as never)` against
unvalidated `searchParams.get('next')`. An attacker could send a
victim to `/portal/login?next=https://evil.example` and the post-sign-in
redirect would navigate cross-site. Add `safeNextPath()` that requires
`/portal/...` prefix and rejects protocol-relative URLs; everything
else falls back to `/portal/dashboard`.

**Other auth-flow items confirmed resolved by earlier waves:**
- H6 resolve-identifier enumeration: endpoint deleted in Wave 1
  (replaced with sign-in-by-identifier which keeps the synthetic
  email behind a server-side proxy)

Tests updated: portal-auth integration test mocks `db` so the new
DB-watermark lookup in `verifyPortalToken` stays unit-pure.

Tests 1315/1315 after `psql ALTER TABLE` to apply migration locally.

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

src/lib/portal/auth.ts

@@ -1,5 +1,9 @@
 import { SignJWT, jwtVerify } from 'jose';
 import { cookies } from 'next/headers';
+import { eq } from 'drizzle-orm';
+
+import { db } from '@/lib/db';
+import { portalUsers } from '@/lib/db/schema/portal';
 
 const PORTAL_SECRET = new TextEncoder().encode(process.env.BETTER_AUTH_SECRET);
 export const PORTAL_COOKIE = 'portal_session';
@@ -14,6 +18,10 @@ export interface PortalSession {
   clientId: string;
   portId: string;
   email: string;
+  /** Portal user id — needed by verifyPortalToken to fetch
+   *  passwordChangedAt for the iat-vs-watermark check. Mirrors what
+   *  `portalUsers.id` resolves to. */
+  portalUserId: string;
 }
 
 export async function createPortalToken(session: PortalSession): Promise<string> {
@@ -32,7 +40,25 @@ export async function verifyPortalToken(token: string): Promise<PortalSession |
       audience: PORTAL_AUD,
       issuer: PORTAL_ISS,
     });
-    return payload as unknown as PortalSession;
+    const session = payload as unknown as PortalSession & { iat?: number };
+
+    // auth-flow-auditor C1 (portal half): reject tokens issued before
+    // the user's last password change so a stolen cookie stops working
+    // after the legitimate owner does the forgot-password dance. The
+    // portalUserId claim is required for the lookup; tokens issued by
+    // the pre-C1 codepath lack it and are rejected on that grounds
+    // alone (forces re-login post-deploy, 24h max delay).
+    if (!session.portalUserId || !session.iat) return null;
+    const user = await db.query.portalUsers.findFirst({
+      where: eq(portalUsers.id, session.portalUserId),
+      columns: { passwordChangedAt: true, isActive: true },
+    });
+    if (!user || !user.isActive) return null;
+    const iatSeconds = session.iat;
+    const watermarkSeconds = Math.floor(user.passwordChangedAt.getTime() / 1000);
+    if (iatSeconds < watermarkSeconds) return null;
+
+    return session;
   } catch {
     return null;
   }