refactor(api-server): extract IMPORTANT_DOC_TYPES constant

Move the duplicated important-document-type list out of routes-tasks.ts
and routes-files.ts into src/shared/document-categories.ts so the array
and lookup set live in one place.

src/shared/document-categories.ts

@@ -0,0 +1,22 @@
+/**
+ * Document type categories shared between db, route handlers, and clients.
+ *
+ * IMPORTANT_DOC_TYPES are the document types that trigger deliverable-review
+ * approvals (PRDs, specs, design docs, contracts, etc.). The ordering matters
+ * for some lookup queries; keep this in sync with database/documents.ts.
+ */
+export const IMPORTANT_DOC_TYPES = [
+  "prd",
+  "technical_spec",
+  "design_doc",
+  "project_brief",
+  "proposal",
+  "budget_proposal",
+  "architecture_decision",
+  "contract",
+  "sop",
+] as const;
+
+export type ImportantDocType = (typeof IMPORTANT_DOC_TYPES)[number];
+
+export const IMPORTANT_DOC_TYPE_SET: ReadonlySet<string> = new Set(IMPORTANT_DOC_TYPES);