feat(web): registration gate + per-request user-id seam (ADR-012)

Multi-user web surface. Two layered middlewares: Auth.Middleware (Dex
session required) wraps registrationGate, which resolves the authenticated
subject -> tapir user_id once per request via the new web.Identity port and
stashes it. A subject with no tapir user is redirected to GET /register
(display name + accept-terms); POST /register calls RegisterUser then
redirects to /. /register is inside the auth guard but exempt from the gate
(/auth/* and /healthz too).

Current-user seam: CurrentUserID(r) (string, bool) returns the resolved id
from the request context. The list/detail/action handlers now scope by it,
replacing the single configured App.UserID (removed). App gains an Identity
field; *store.Store satisfies both Store and Identity. cmd/tapir wires
Identity: st and drops UserID.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/views.templ

@@ -153,6 +153,35 @@ templ DetailPage(r store.SummaryRow) {
 	}
 }
 
+// RegisterPage is the explicit registration step (ADR-012): an authenticated Dex
+// subject with no tapir user picks a display name and accepts the terms to create
+// their account. errMsg, when set, reports a validation problem on the prior POST.
+templ RegisterPage(email, errMsg string) {
+	@Layout("Tapir — Register") {
+		<article class="register">
+			<h1>Complete your registration</h1>
+			if email != "" {
+				<p class="meta">Signed in as { email }.</p>
+			}
+			<p>Choose a display name to finish setting up your Tapir account.</p>
+			if errMsg != "" {
+				<p class="error" role="alert">{ errMsg }</p>
+			}
+			<form method="post" action="/register" class="register-form">
+				<label>
+					Display name
+					<input type="text" name="display_name" required autofocus/>
+				</label>
+				<label class="checkbox">
+					<input type="checkbox" name="accept_terms" value="yes" required/>
+					I accept the terms of use
+				</label>
+				<button type="submit" class="btn">Register</button>
+			</form>
+		</article>
+	}
+}
+
 // ActionButtons is the toggle group fragment returned by POST /v/{id}/action.
 // Each button submits its verb; HTMX swaps this element in place (outerHTML),
 // and without JS the form POSTs and the handler redirects back to the detail