Build the tenant-picker backend protocol (no frontend yet, by design)

A multi-membership identity (belongs to more than one tenant) used to
get a flat 501 refusal -- named as undesigned future work across
CLAUDE.md/threat-model.md/the runbook since early Phase 4. Scope for
this change was agreed via AskUserQuestion: backend protocol only,
fully verified via real HTTP round trips, not the actual picker page --
web has zero session/cookie-handling code today (confirmed while
researching this), so building that is separately-scoped, unverifiable
frontend work in this environment (no live backend, no browser).

session.Manager gains IssuePendingLogin/ValidatePendingLogin, a second
JWT token type proving identity without committing to a tenant yet
(10-minute TTL). PendingLoginClaims is deliberately a distinct Go type
from Claims, and -- caught by this change's own test suite before it
shipped -- needed a JSON field name disjoint from Claims.UserID's
"user_id" too: go-jose's unmarshal is happy to populate a struct from
any token whose claims happen to share a key, so a real session token
would otherwise have parsed successfully as a pending login. Fixed via
"pending_user_id" instead; both directions (session-as-pending,
pending-as-session) now have regression tests.

rbacstore.ListMembershipsWithTenantForUser joins tenant_memberships
with tenants, since a picker needs display names, not just IDs.

loginhandler.resolveIdentity's multiple-membership branch no longer
errors -- finishLogin routes it into startTenantSelection instead,
which issues a pending-login cookie (Path=/auth, so it's never sent on
ordinary requests) and redirects to a new configurable
SelectTenantRedirectURL (defaults to {POST_LOGIN_REDIRECT_URL}/select-
tenant). Two new routes complete the round trip: GET /auth/memberships
lists the pending identity's real tenant options, and POST
/auth/select-tenant re-derives the role for the chosen tenant
server-side (never trusts a client-supplied role, refuses a tenant_id
outside the identity's actual memberships with 403) before issuing the
real session -- responding with JSON {"redirect_url": ...}, not a
redirect, since a POST/fetch caller should control its own navigation.

Verified with the same real-fake-IdP tests the rest of this package
uses (coreos/go-oidc's oidctest, crewjam/saml's samlidp): the full
login -> pending cookie -> GET /auth/memberships -> POST
/auth/select-tenant -> real session round trip for both protocols, plus
negative paths (missing/expired pending cookie, a tenant_id outside
membership, a real session token rejected as a pending login and vice
versa). ErrMultipleMemberships is removed -- it's not an error path
anymore.

Docs updated in lockstep: CLAUDE.md, threat-model.md (including its
summary table), phase-4-runbook.md (new §12), enterprise/README.md
(new "Tenant selection" section, explicit about what's still not built
and why: no session handling in web, no CORS on enterprise-auth).

enterprise/internal/config/config.go

@@ -17,6 +17,16 @@ type Config struct {
 	// internal/loginhandler sets a session cookie -- web's base URL in
 	// a real deployment.
 	PostLoginRedirectURL string
+	// SelectTenantRedirectURL is where the browser lands after a login
+	// resolves to more than one tenant_memberships row --
+	// internal/loginhandler issues a pending-login cookie and sends the
+	// browser here instead of straight to PostLoginRedirectURL. Nothing
+	// serves this route yet (a real tenant-picker page is undesigned
+	// frontend work -- see internal/loginhandler's package doc comment);
+	// the backend protocol (GET /auth/memberships, POST
+	// /auth/select-tenant) is complete and independently testable via
+	// HTTP regardless of what, if anything, is listening here today.
+	SelectTenantRedirectURL string
 }
 
 type PostgresConfig struct {
@@ -70,6 +80,12 @@ func Load() (Config, error) {
 		},
 		PostLoginRedirectURL: getenv("POST_LOGIN_REDIRECT_URL", "http://localhost:3000"),
 	}
+	// Defaults relative to PostLoginRedirectURL if not set explicitly --
+	// computed after cfg.PostLoginRedirectURL above so a caller
+	// overriding just POST_LOGIN_REDIRECT_URL still gets a sensible
+	// SelectTenantRedirectURL without also having to set the new
+	// variable.
+	cfg.SelectTenantRedirectURL = getenv("SELECT_TENANT_REDIRECT_URL", cfg.PostLoginRedirectURL+"/select-tenant")
 
 	// Required, unlike OIDC/SAML above: every enterprise-auth deployment
 	// issues and validates session/service tokens (internal/session),