feat(share): custom /l/:alias pretty links (share_aliases table) (#205)

Add a retargetable, human-readable vanity link namespace /l/<alias> that
sits alongside the untouched /share/... routes.

- New share_aliases table (workspace-scoped, UNIQUE(workspace_id, alias),
  page_id nullable ON DELETE SET NULL so the address outlives its target).
- ShareAliasRepo + ShareAliasService (create / no-op / 409 reassign guard /
  availability / request-time readable-target resolution through the single
  existing share boundary).
- Public ShareAliasRedirectController (GET /l/:alias) issues a 302 (never 301,
  the target is mutable) to the canonical /share/:key/p/:slug page; unknown /
  dangling / no-longer-readable aliases serve the SPA index with no leak.
  'l/:alias' excluded from the global /api prefix.
- Authenticated ShareAliasController (set/remove/availability/for-page).
- Shared ASCII-only normalize/validate util (server + client copies).
- Client: Custom address block in the share modal (live normalize + debounced
  availability + copy + reassign confirmation dialog).
- Unit tests: util, repo SQL-shape, service semantics, migration/entity sanity
  (server jest) + client alias util (vitest).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/main.ts

@@ -40,7 +40,14 @@ async function bootstrap() {
   app.useLogger(app.get(PinoLogger));
 
   app.setGlobalPrefix('api', {
-    exclude: ['robots.txt', 'share/:shareId/p/:pageSlug', 'mcp'],
+    exclude: [
+      'robots.txt',
+      'share/:shareId/p/:pageSlug',
+      // Vanity link resolver lives outside /api so /l/<alias> is a clean
+      // public URL that 302s to the canonical share page.
+      'l/:alias',
+      'mcp',
+    ],
   });
 
   const reflector = app.get(Reflector);