fix(security): promote auth_token cookie to Authorization header for browser /api/* calls

Closes #520.

The login action stores `Basic <base64>` in an HttpOnly `auth_token`
cookie. SSR fetches from hooks.server.ts explicitly set the
Authorization header. Vite's dev proxy does the same on every
/api/* request. Caddy in production does NOT. So browser-side
fetch() and EventSource() calls reach the backend without auth,
get 401 + WWW-Authenticate: Basic, and the browser pops a native
auth dialog over the SPA.

Add AuthTokenCookieFilter (Ordered.HIGHEST_PRECEDENCE, before any
Spring Security filter) that promotes the cookie to a request
header when no explicit Authorization is present. URL-decodes the
cookie value because SvelteKit URL-encodes spaces ("Basic " ->
"Basic%20") when serializing the cookie. Works the same for REST,
SSE (/api/notifications/stream, /api/ocr/jobs/.../progress), and
any other browser-direct backend call.

5 tests in AuthTokenCookieFilterTest cover: URL-decoded promotion,
explicit-Authorization-wins precedence, no-cookies pass-through,
absent-auth-token pass-through, empty-value pass-through.

Also: add `@ActiveProfiles("test")` to ThumbnailServiceIntegrationTest,
the one remaining @SpringBootTest in the suite that wasn't annotated.
After #516 made UserDataInitializer fail-closed outside dev/test/e2e,
this test's context load was throwing. Restores green main.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/security/AuthTokenCookieFilter.java

@@ -0,0 +1,109 @@
+package org.raddatz.familienarchiv.security;
+
+import jakarta.servlet.FilterChain;
+import jakarta.servlet.ServletException;
+import jakarta.servlet.http.Cookie;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletRequestWrapper;
+import jakarta.servlet.http.HttpServletResponse;
+import org.springframework.core.annotation.Order;
+import org.springframework.http.HttpHeaders;
+import org.springframework.stereotype.Component;
+import org.springframework.web.filter.OncePerRequestFilter;
+
+import java.io.IOException;
+import java.net.URLDecoder;
+import java.nio.charset.StandardCharsets;
+import java.util.Collections;
+import java.util.Enumeration;
+
+/**
+ * Promotes the {@code auth_token} cookie to an {@code Authorization} header
+ * so that browser-side requests to {@code /api/*} authenticate the same way
+ * SSR fetches do.
+ *
+ * <p>The SvelteKit login action stores the full HTTP Basic header value
+ * ({@code "Basic <base64>"}) in an HttpOnly cookie. SSR fetches from
+ * {@code hooks.server.ts} read the cookie and pass it explicitly as the
+ * {@code Authorization} header. In the dev environment, Vite's proxy does
+ * the same on every {@code /api/*} request (see {@code vite.config.ts}).
+ * In production, Caddy proxies {@code /api/*} straight to the backend and
+ * does NOT translate the cookie — so client-side {@code fetch} and
+ * {@code EventSource} calls reach the backend without auth, get
+ * {@code 401 WWW-Authenticate: Basic}, and the browser pops a native dialog.
+ *
+ * <p>This filter closes that gap: if a request has an {@code auth_token}
+ * cookie but no explicit {@code Authorization} header, promote the cookie
+ * value (URL-decoded) into the header before Spring Security inspects it.
+ * Explicit {@code Authorization} headers are preserved unchanged.
+ *
+ * <p>See #520. Filter runs at {@code Ordered.HIGHEST_PRECEDENCE} so it
+ * mutates the request before any Spring Security filter sees it.
+ */
+@Component
+@Order(org.springframework.core.Ordered.HIGHEST_PRECEDENCE)
+public class AuthTokenCookieFilter extends OncePerRequestFilter {
+
+    static final String COOKIE_NAME = "auth_token";
+
+    @Override
+    protected void doFilterInternal(HttpServletRequest request,
+                                    HttpServletResponse response,
+                                    FilterChain chain) throws ServletException, IOException {
+        if (request.getHeader(HttpHeaders.AUTHORIZATION) != null) {
+            chain.doFilter(request, response);
+            return;
+        }
+        Cookie[] cookies = request.getCookies();
+        if (cookies == null) {
+            chain.doFilter(request, response);
+            return;
+        }
+        for (Cookie c : cookies) {
+            if (COOKIE_NAME.equals(c.getName()) && c.getValue() != null && !c.getValue().isEmpty()) {
+                String decoded = URLDecoder.decode(c.getValue(), StandardCharsets.UTF_8);
+                chain.doFilter(new AuthHeaderRequest(request, decoded), response);
+                return;
+            }
+        }
+        chain.doFilter(request, response);
+    }
+
+    /**
+     * Adds (or overrides) the {@code Authorization} header on a wrapped request.
+     * All other headers pass through unchanged.
+     */
+    static final class AuthHeaderRequest extends HttpServletRequestWrapper {
+        private final String authorization;
+
+        AuthHeaderRequest(HttpServletRequest request, String authorization) {
+            super(request);
+            this.authorization = authorization;
+        }
+
+        @Override
+        public String getHeader(String name) {
+            if (HttpHeaders.AUTHORIZATION.equalsIgnoreCase(name)) {
+                return authorization;
+            }
+            return super.getHeader(name);
+        }
+
+        @Override
+        public Enumeration<String> getHeaders(String name) {
+            if (HttpHeaders.AUTHORIZATION.equalsIgnoreCase(name)) {
+                return Collections.enumeration(Collections.singletonList(authorization));
+            }
+            return super.getHeaders(name);
+        }
+
+        @Override
+        public Enumeration<String> getHeaderNames() {
+            Enumeration<String> base = super.getHeaderNames();
+            java.util.Set<String> names = new java.util.LinkedHashSet<>();
+            while (base.hasMoreElements()) names.add(base.nextElement());
+            names.add(HttpHeaders.AUTHORIZATION);
+            return Collections.enumeration(names);
+        }
+    }
+}