Apply public-content-visibility: visibility flag + public profile

Implements the public-content-visibility OpenSpec change. Adds the
smallest social surface that lets us demo the product to logged-out
visitors without user signup.

Schema:
- `visibility text NOT NULL DEFAULT 'private'` on routes + activities.
- Shared Visibility type exported from the schema module.

Access:
- New canView(content, viewer, { asDirectLink }) helper in auth.server.ts
  centralises the rule: public → anyone; unlisted → anyone on direct
  link; private → owner only.
- routes.$id and activities.$id loaders return 404 (not 403) when
  canView rejects, so existence of private content isn't leaked.
- Detail pages emit Open Graph + Twitter Card meta on public/unlisted
  content only.

Editing:
- Visibility <select> on routes/:id/edit with owner-only access.
- Activity detail page gets a small visibility form + set-visibility
  action intent (no separate activity-edit page needed).
- EN + DE i18n under routes.visibility.* and activities.visibility.*.

Listings:
- Listing helpers listPublicRoutesForOwner / listPublicActivitiesForOwner
  for cross-user queries. Existing owner-scoped listRoutes/listActivities
  stay — owners see their own content regardless of visibility.

Public profile:
- /users/:username is now truly public. Renders the user's public
  routes + activities, 404s when no public content exists AND viewer
  isn't the owner (prevents account enumeration).
- Owner sees a short "this is your profile" note linking to settings.
- Open Graph meta (og:type=profile) for shareable preview.

Privacy manifest:
- Added a bullet noting public content is world-visible on profile
  and indexable by search engines.
- Bumped PRIVACY_LAST_UPDATED to 2026-04-20 + rendered legal-archive
  snapshot.

Tests:
- 13 unit tests for canView covering the full matrix.
- 6 E2E tests in e2e/public-content.test.ts covering:
  - Private route → 404 for logged-out visitor
  - Public route → reachable + OG tags present (og:title, og:type,
    og:site_name)
  - Owner still sees own private content
  - Profile 404 when no public content
  - Profile renders when at least one public route exists
  - Unlisted route reachable via direct URL but hidden from profile
- Test file runs serially (describe.configure mode=serial) to avoid
  WebAuthn virtual-authenticator races under Playwright's default
  parallel workers.
- New public-content Playwright project added to config.

Rollout safety: every existing row in prod keeps visibility='private'
by default — nothing becomes visible to outsiders until an owner
explicitly opts in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

playwright.config.ts

@@ -39,6 +39,14 @@ export default defineConfig({
         baseURL: "http://localhost:3000",
       },
     },
+    {
+      name: "public-content",
+      testMatch: "public-content.test.ts",
+      use: {
+        ...devices["Desktop Chrome"],
+        baseURL: "http://localhost:3000",
+      },
+    },
     {
       name: "integration",
       testMatch: "integration.test.ts",