feat: wire auth into the web layer (Schritt 2, part 2/2)

Registration creates a new account plus its first user; login
authenticates an existing one; both set a deklarix_session cookie
(HttpOnly, SameSite=Strict, Secure only when the request itself came
over TLS — hardcoding Secure=true would break local http://localhost
development, since browsers won't store a Secure cookie over plaintext).

requirePage protects full-page GETs (redirects to /login); requireAPI
protects the htmx/download endpoints (401, since those are only ever
called from an already-authenticated page — an unauthenticated hit
there is the exception, e.g. a session expiring mid-use).

handleCheck now creates submissions under the current account.
handleArchive and handleDossierDownload compare the submission's
account against the caller's and return 404 on mismatch — not 403,
which would confirm the ID exists to a different tenant. Login failure
uses the same message for "no such email" and "wrong password" to avoid
account enumeration.

Restructured templates along the way: layout.html now only holds
reusable fragments ("head", "nav"); each full page (index/login/register)
is its own top-level named template. The previous layout+content nesting
would have broken the moment a second page defined "content" — Go's
html/template keys blocks by name across the whole parsed set, not per
file, so two pages both defining "content" would silently overwrite each
other.

Verified against a real running instance (not just Go's test recorder):
started the compiled binary against a fresh Postgres and drove the whole
flow with curl — anonymous redirect, registration setting a real cookie,
authenticated page load, logout clearing both the cookie and the
server-side session row, and being locked out again afterward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/web/middleware.go

@@ -0,0 +1,81 @@
+package web
+
+import (
+	"context"
+	"net/http"
+	"time"
+
+	"github.com/netcell-it/deklarix/internal/store"
+)
+
+const sessionCookieName = "deklarix_session"
+
+type contextKey int
+
+const userContextKey contextKey = iota
+
+// authenticate liest das Session-Cookie, prüft die Sitzung (existiert,
+// nicht abgelaufen) und lädt den zugehörigen Nutzer. Liefert (User{}, false),
+// wenn irgendein Schritt fehlschlägt — die Gründe (kein Cookie, unbekanntes
+// Token, abgelaufen, Nutzer weg) werden bewusst nicht unterschieden, damit
+// requirePage/requireAPI immer denselben, einzigen Fehlerpfad haben.
+func (s *Server) authenticate(r *http.Request) (store.User, bool) {
+	cookie, err := r.Cookie(sessionCookieName)
+	if err != nil || cookie.Value == "" {
+		return store.User{}, false
+	}
+
+	sess, err := s.store.GetSession(r.Context(), cookie.Value)
+	if err != nil {
+		return store.User{}, false
+	}
+	if time.Now().After(sess.ExpiresAt) {
+		return store.User{}, false
+	}
+
+	user, err := s.store.GetUser(r.Context(), sess.UserID)
+	if err != nil {
+		return store.User{}, false
+	}
+	return user, true
+}
+
+// requirePage schützt volle Seitenaufrufe — ohne gültige Sitzung geht
+// es zurück zu /login (eine echte Navigation, kein htmx-Fragment).
+func (s *Server) requirePage(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		user, ok := s.authenticate(r)
+		if !ok {
+			http.Redirect(w, r, "/login", http.StatusSeeOther)
+			return
+		}
+		next(w, r.WithContext(context.WithValue(r.Context(), userContextKey, user)))
+	}
+}
+
+// requireAPI schützt htmx-Endpunkte (Formular-Posts, Downloads) — diese
+// werden nur aus einer bereits authentifizierten Seite heraus
+// aufgerufen, ein Fehlschlag hier ist der Ausnahmefall (z. B. Sitzung
+// mitten in der Nutzung abgelaufen), daher schlicht 401 statt Redirect.
+func (s *Server) requireAPI(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		user, ok := s.authenticate(r)
+		if !ok {
+			http.Error(w, "nicht angemeldet", http.StatusUnauthorized)
+			return
+		}
+		next(w, r.WithContext(context.WithValue(r.Context(), userContextKey, user)))
+	}
+}
+
+// currentUser liest den Nutzer, den requirePage/requireAPI in den
+// Kontext gelegt haben. Panics, wenn es aufgerufen wird, ohne dass eine
+// dieser Middlewares vorgeschaltet war — das ist ein Programmierfehler,
+// kein Laufzeitfall, den man abfangen sollte.
+func currentUser(r *http.Request) store.User {
+	user, ok := r.Context().Value(userContextKey).(store.User)
+	if !ok {
+		panic("web: currentUser aufgerufen ohne requirePage/requireAPI")
+	}
+	return user
+}