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/database/database.module.ts

@@ -23,6 +23,7 @@ import { UserTokenRepo } from './repos/user-token/user-token.repo';
 import { UserSessionRepo } from '@docmost/db/repos/session/user-session.repo';
 import { BacklinkRepo } from '@docmost/db/repos/backlink/backlink.repo';
 import { ShareRepo } from '@docmost/db/repos/share/share.repo';
+import { ShareAliasRepo } from '@docmost/db/repos/share-alias/share-alias.repo';
 import { NotificationRepo } from '@docmost/db/repos/notification/notification.repo';
 import { WatcherRepo } from '@docmost/db/repos/watcher/watcher.repo';
 import { LabelRepo } from '@docmost/db/repos/label/label.repo';
@@ -96,6 +97,7 @@ import { normalizePostgresUrl } from '../common/helpers';
     UserSessionRepo,
     BacklinkRepo,
     ShareRepo,
+    ShareAliasRepo,
     NotificationRepo,
     WatcherRepo,
     LabelRepo,
@@ -128,6 +130,7 @@ import { normalizePostgresUrl } from '../common/helpers';
     UserSessionRepo,
     BacklinkRepo,
     ShareRepo,
+    ShareAliasRepo,
     NotificationRepo,
     WatcherRepo,
     LabelRepo,