fix: persist Ed25519 signing key to survive server restarts

The keypair was generated ephemerally on each startup, causing agents
to reject all commands after a server restart (signature mismatch).
Now persisted to PostgreSQL server_config table and restored on startup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/test/java/com/cameleer3/server/app/agent/SsePayloadSignerTest.java

@@ -29,7 +29,7 @@ class SsePayloadSignerTest {
 
     @BeforeEach
     void setUp() {
-        signingService = new Ed25519SigningServiceImpl();
+        signingService = Ed25519SigningServiceImpl.ephemeral();
         objectMapper = new ObjectMapper();
         signer = new SsePayloadSigner(signingService, objectMapper);
     }

cameleer3-server-app/src/test/java/com/cameleer3/server/app/security/Ed25519SigningServiceTest.java

@@ -22,7 +22,7 @@ class Ed25519SigningServiceTest {
 
     @BeforeEach
     void setUp() {
-        signingService = new Ed25519SigningServiceImpl();
+        signingService = Ed25519SigningServiceImpl.ephemeral();
     }
 
     @Test