feat(memo): add and delete sections via RPC

Add memo.addSection and memo.deleteSection RPC methods with full-stack
implementation: contract types, Zod schemas, DB queries, RPC handlers,
UI buttons, and tests.

- memo.addSection: creates a new empty section, supports positional
  insert after a given section via afterSectionId
- memo.deleteSection: removes a section and cascades cleanup of its
  annotations, citations, and section reviews
- Outline sidebar: per-section delete button, + Add Section button
- Article area: + Add Section button after last section
- 9 new tests (15 total memo tests, 48 project-wide)

packages/contracts/src/rpc.ts

@@ -211,6 +211,15 @@ export type RpcRequestMap = {
   "model.deleteRow": { companyId: string; tab: string; row: number };
   "model.runScenario": { companyId: string; scenario: string; overrides: Record<string, string> };
   "memo.get": { companyId: string };
+  "memo.addSection": {
+    companyId: string;
+    title: string;
+    afterSectionId?: string;
+  };
+  "memo.deleteSection": {
+    companyId: string;
+    sectionId: string;
+  };
   "memo.updateSection": {
     companyId: string;
     sectionId: string;
@@ -282,6 +291,12 @@ export type RpcResponseMap = {
     annotations: MemoAnnotation[];
     sectionReviews: MemoSectionReview[];
   };
+  "memo.addSection": {
+    section: MemoSection;
+  };
+  "memo.deleteSection": {
+    ok: boolean;
+  };
   "memo.updateSection": {
     section: MemoSection;
     status: "draft" | "review" | "final";