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/db/schema/portal.ts

@@ -29,6 +29,17 @@ export const portalUsers = pgTable(
      * until the user activates their account.
      */
     passwordHash: text('password_hash'),
+    /**
+     * Watermark for JWT-session revocation on password change. Any
+     * `verifyPortalToken` call where the JWT's `iat` is older than this
+     * value rejects the token even if it's otherwise valid. Updated on
+     * `resetPassword`, `activateAccount`, and `changePortalPassword` so
+     * a stolen cookie stops working after the legitimate owner does the
+     * forgot-password / change-password dance. auth-flow-auditor C1.
+     */
+    passwordChangedAt: timestamp('password_changed_at', { withTimezone: true })
+      .notNull()
+      .defaultNow(),
     name: text('name'),
     isActive: boolean('is_active').notNull().default(true),
     lastLoginAt: timestamp('last_login_at', { withTimezone: true }),