Fix RFC 1123 date format - use zero-padded day

The issue was that Java's DateTimeFormatter.RFC_1123_DATE_TIME produces
dates like 'Tue, 2 Dec 2025' (without zero-padded day) but Mastodon
requires strict RFC 1123 format with zero-padded day: 'Tue, 02 Dec 2025'.

This was causing HTTP Signature validation failures (401 Unauthorized)
because the Date header in the signed string didn't match the actual header.

Changes:
- Fixed date format in HttpSignatureValidator to use custom pattern
- Pattern: 'EEE, dd MMM yyyy HH:mm:ss GMT' with Locale.US
- Added date format test to verify correct output
- Added debug endpoint for key validation
- Explicitly set Host header in FederationService

This should fix the 401 errors when federating with Mastodon.

src/test/java/org/operaton/fitpub/security/DateFormatTest.java

@@ -0,0 +1,31 @@
+package org.operaton.fitpub.security;
+
+import org.junit.jupiter.api.Test;
+
+import java.time.ZoneOffset;
+import java.time.ZonedDateTime;
+import java.time.format.DateTimeFormatter;
+
+public class DateFormatTest {
+
+    @Test
+    public void testRFC1123DateFormat() {
+        ZonedDateTime now = ZonedDateTime.now(ZoneOffset.UTC);
+
+        // Old format (broken)
+        DateTimeFormatter oldFormatter = DateTimeFormatter.RFC_1123_DATE_TIME;
+        String oldDate = now.format(oldFormatter);
+        System.out.println("OLD RFC 1123 Date (broken): " + oldDate);
+
+        // New format (correct)
+        DateTimeFormatter newFormatter = DateTimeFormatter.ofPattern(
+            "EEE, dd MMM yyyy HH:mm:ss 'GMT'",
+            java.util.Locale.US
+        );
+        String newDate = now.format(newFormatter);
+        System.out.println("NEW RFC 1123 Date (correct): " + newDate);
+
+        // Mastodon expects format like: "Mon, 02 Dec 2024 20:48:33 GMT"
+        // Note the zero-padded day "02" not "2"
+    }
+}