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/client/src/features/share/services/share-service.ts

@@ -4,6 +4,9 @@ import { IPage } from "@/features/page/types/page.types";
 import {
   ICreateShare,
   IShare,
+  IShareAlias,
+  IShareAliasAvailability,
+  ISetShareAlias,
   ISharedItem,
   ISharedPage,
   ISharedPageTree,
@@ -57,3 +60,33 @@ export async function getSharedPageTree(
   const req = await api.post<ISharedPageTree>("/shares/tree", { shareId });
   return req.data;
 }
+
+export async function getShareAliasForPage(
+  pageId: string,
+): Promise<IShareAlias | null> {
+  const req = await api.post<IShareAlias | null>("/share-aliases/for-page", {
+    pageId,
+  });
+  return req.data;
+}
+
+export async function setShareAlias(
+  data: ISetShareAlias,
+): Promise<IShareAlias> {
+  const req = await api.post<IShareAlias>("/share-aliases/set", data);
+  return req.data;
+}
+
+export async function removeShareAlias(aliasId: string): Promise<void> {
+  await api.post("/share-aliases/remove", { aliasId });
+}
+
+export async function checkShareAliasAvailability(
+  alias: string,
+): Promise<IShareAliasAvailability> {
+  const req = await api.post<IShareAliasAvailability>(
+    "/share-aliases/availability",
+    { alias },
+  );
+  return req.data;
+}