feat: revamp to dynamic database-first architecture and cleanup phase docs

- Migrate document fetching and CRUD to be PostgreSQL-authoritative
- Remove static section enums and add dynamic listSections query
- Support custom sections and metadata across API, MCP, and Web UI
- Add /api/sections endpoint and update Header, Sidebar, and Forms
- Remove obsolete phase planning docs and modernize README/AGENTS

packages/core/src/index.ts

@@ -12,4 +12,6 @@ export type {
   Document,
   DocumentMeta,
   SearchHit,
+  SectionInfo,
 } from "@mcpedia/types";
+