refactor(import): remove non-functional DOCX/PDF/Confluence import stubs

These import paths relied on the private EE module that was deleted from
the repo. In the community build they either threw 'enterprise license'
(DOCX/PDF) or silently no-op'd (Confluence). The frontend buttons were
already removed in 38064064; this cleans up the dead backend stubs.

- import.service.ts: drop processDocx/processPdf methods, their dispatcher
  branches, the pageId computation + insertPage spread, and the now-unused
  moduleRef param/ModuleRef import
- file-import-task.service.ts: drop the Confluence branch and the now-unused
  moduleRef param/ModuleRef import
- import.controller.ts: restrict file extensions to .md/.html and zip
  sources to generic/notion; update the error message accordingly
- file.utils.ts: remove Confluence from the FileImportSource enum
- features.ts: remove the unused CONFLUENCE_IMPORT/DOCX_IMPORT/PDF_IMPORT
  feature keys

The isConfluenceImport logic in import-attachment.service.ts is intentionally
left in place (real shared attachment-parsing code, not a stub); its removal
is a separate, riskier refactor.

apps/server/src/common/features.ts

@@ -6,9 +6,6 @@ export const Feature = {
   COMMENT_RESOLUTION: 'comment:resolution',
   PAGE_PERMISSIONS: 'page:permissions',
   AI: 'ai',
-  CONFLUENCE_IMPORT: 'import:confluence',
-  DOCX_IMPORT: 'import:docx',
-  PDF_IMPORT: 'import:pdf',
   ATTACHMENT_INDEXING: 'attachment:indexing',
   SECURITY_SETTINGS: 'security:settings',
   MCP: 'mcp',