Preserve deleted server history

Retain historical server identities, results, and evidence after deletion. Mark archived entries as Deleted and prevent old work from targeting servers that reuse the same name.

Fixes #16.

tests/unit/watchdog.test.ts

@@ -195,7 +195,7 @@ describe('Watchdog detection and ownership', () => {
 });
 
 describe('Watchdog recovery persistence', () => {
-    it('resolves an attention incident after an intentional successful destroy', async () => {
+    it('preserves the original finding as deleted history after an intentional successful destroy', async () => {
         const { removeServerKey } = await import('../../src/lib/server/fleet/server-keys.js');
         const { destroyServer, processDestructions } = await import('../../src/lib/server/provisioning/instances.js');
         removeServerKey(server);
@@ -209,8 +209,8 @@ describe('Watchdog recovery persistence', () => {
         expect(operations()).toEqual([expect.objectContaining({ action: 'destroy', state: 'done' })]);
         expect(database.db().prepare('SELECT name FROM servers WHERE name = ?').get(server)).toBeUndefined();
         await watchdog.watchdogTick(true);
-        expect(watchdog.watchdogDetail(id)?.incident).toMatchObject({ state: 'resolved', finished_at: expect.any(String) });
-        expect(watchdog.watchdogDetail(id)?.events.at(-1)?.message).toMatch(/destroyed|registered|retired/i);
+        expect(watchdog.watchdogDetail(id)?.incident).toMatchObject({ state: 'attention', server_deleted_at: expect.any(String) });
+        expect(watchdog.updateWatchdogIncident(id, 'retry')).toMatchObject({ ok: false, status: 409 });
     });
 
     it('requests one restart, escalates once to rebuild, and retains exhausted attention across reopening', async () => {