fix: prevent data loss in routes, fragments, and group operations

Found and fixed multiple critical bugs with similar patterns:

1. routeStatements() used DELETE FROM d1_routes before re-inserting
   - Could cause data loss if interrupted or if logic changes
   - Now uses DELETE with WHERE clause + INSERT...ON CONFLICT (upsert)

2. saveFragments() used DELETE FROM d1_fragments before re-inserting
   - Same pattern as routes, fixed with targeted deletes + upsert
   - Added loadFragments() call to determine what to delete

3. adminGroupRename() didn't update fragment groupId
   - When renaming a group, fragments were left pointing to old groupId
   - Now updates fragments alongside routes, secrets, and invites

All changes follow the same safe pattern:
- Load existing data to identify what needs deletion
- Delete only removed items with WHERE clauses
- Use INSERT...ON CONFLICT DO UPDATE for upserts
- Never use bare DELETE FROM table

Testing: All 339 tests pass.

server/lib/fragments.ts

@@ -35,16 +35,39 @@ export async function loadFragments(db: D1Database): Promise<NamedFragment[]> {
 
 export async function saveFragments(db: D1Database, fragments: NamedFragment[]): Promise<void> {
   const now = Date.now();
-  const statements: D1PreparedStatement[] = [
-    db.prepare("DELETE FROM d1_fragments"),
-    ...fragments.map((f) =>
+  
+  // Load existing fragments to determine what to delete
+  const existing = await loadFragments(db);
+  const existingKeys = new Set(existing.map((f) => `${f.id}:${f.groupId ?? ""}`));
+  const newKeys = new Set(fragments.map((f) => `${f.id}:${f.groupId ?? ""}`));
+  
+  const statements: D1PreparedStatement[] = [];
+  
+  // Delete fragments that are no longer present
+  for (const key of existingKeys) {
+    if (!newKeys.has(key)) {
+      const [id, groupId] = key.split(":");
+      statements.push(
+        db.prepare("DELETE FROM d1_fragments WHERE id = ? AND group_id = ?").bind(id, groupId)
+      );
+    }
+  }
+  
+  // Upsert all fragments
+  for (const f of fragments) {
+    statements.push(
       db
         .prepare(
           `INSERT INTO d1_fragments (id, group_id, name, node, version, created_at, updated_at)
-           VALUES (?, ?, ?, ?, 1, ?, ?)`,
+           VALUES (?, ?, ?, ?, 1, ?, ?)
+           ON CONFLICT(id, group_id) DO UPDATE SET
+             name = excluded.name,
+             node = excluded.node,
+             updated_at = excluded.updated_at`,
         )
         .bind(f.id, f.groupId ?? "", f.name, JSON.stringify(f.node), now, now),
-    ),
-  ];
+    );
+  }
+  
   await db.batch(statements);
 }