Add SSO authorization code flow (Phase 1)

MCIAS now acts as an SSO provider for downstream services. Services
redirect users to /sso/authorize, MCIAS handles login (password, TOTP,
or passkey), then redirects back with an authorization code that the
service exchanges for a JWT via POST /v1/sso/token.

- Add SSO client registry to config (client_id, redirect_uri,
  service_name, tags) with validation
- Add internal/sso package: authorization code and session stores
  using sync.Map with TTL, single-use LoadAndDelete, cleanup goroutines
- Add GET /sso/authorize endpoint (validates client, creates session,
  redirects to /login?sso=<nonce>)
- Add POST /v1/sso/token endpoint (exchanges code for JWT with policy
  evaluation using client's service_name/tags from config)
- Thread SSO nonce through password→TOTP and WebAuthn login flows
- Update login.html, totp_step.html, and webauthn.js for SSO nonce
  passthrough

Security:
- Authorization codes are 256-bit random, single-use, 60-second TTL
- redirect_uri validated as exact match against registered config
- Policy context comes from MCIAS config, not the calling service
- SSO sessions are server-side only; nonce is the sole client-visible value
- WebAuthn SSO returns redirect URL as JSON (not HTTP redirect) for JS compat

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/fragments/totp_step.html

@@ -5,6 +5,7 @@
   <input type="hidden" name="username" value="{{.Username}}">
   <input type="hidden" name="totp_nonce" value="{{.Nonce}}">
   <input type="hidden" name="totp_step" value="1">
+  {{if .SSONonce}}<input type="hidden" name="sso_nonce" value="{{.SSONonce}}">{{end}}
   <div class="form-group">
     <label for="totp_code">Authenticator Code</label>
     <input class="form-control" type="text" id="totp_code" name="totp_code"

web/templates/login.html

@@ -15,6 +15,7 @@
       {{if .Error}}<div class="alert alert-error" role="alert">{{.Error}}</div>{{end}}
       <form id="login-form" method="POST" action="/login"
             hx-post="/login" hx-target="#login-card" hx-swap="outerHTML" hx-select="#login-card">
+        {{if .SSONonce}}<input type="hidden" name="sso_nonce" value="{{.SSONonce}}">{{end}}
         <div class="form-group">
           <label for="username">Username</label>
           <input class="form-control" type="text" id="username" name="username"
@@ -37,7 +38,8 @@
           <hr style="flex:1;border:0;border-top:1px solid #ddd">
         </div>
         <div id="webauthn-login-error" class="alert alert-error" style="display:none" role="alert"></div>
-        <button class="btn btn-secondary" type="button" id="webauthn-login-btn" style="width:100%">
+        <button class="btn btn-secondary" type="button" id="webauthn-login-btn" style="width:100%"
+                {{if .SSONonce}}data-sso-nonce="{{.SSONonce}}"{{end}}>
           Sign in with passkey
         </button>
       </div>