Add Happy 404, featured/related sidebars, and next-post nav to the post page

Happy 404: an HTML 404 (unmatched route, or an explicit not-found inside a
real route) now renders a random existing post instead of the error page,
using the exact same context builder as a real post view. Toggle is the
happy_404_enabled site setting (default on, /admin/settings); JSON/API
requests and a handful of excluded prefixes are never affected. The pool of
candidate slugs is cached in-process and resampled periodically so it stays
fast and eventually cycles the whole posts table; on any internal failure it
falls straight through to the real 404 page.

Applying this everywhere surfaced ~60 existing tests that asserted a literal
404 for a legitimate resource-not-found flow (deleted post, unknown
container, wrong project slug, etc.) - each now disables the setting for the
duration of that specific check and restores it after, so the underlying
not-found behavior stays covered independently of the new feature.

Post page also gained, all built on the same shared post_page_context() so
they render identically on both a real post and a happy-404 page:
- A left sidebar (three separate cards, matching /feed's sidebar-card
  convention) for "Gists from {author}", "Projects from {author}" (private
  projects filtered through the normal visibility check), and "Related
  Discussions" - each cached per author and invalidated on create/edit/
  delete so new content shows up immediately.
- A right column reusing /feed's exact Daily Topic widget class for up to
  three "Featured" articles (the existing but previously-unused `featured`
  news flag), cached as a pool with per-request random sampling.
- A "Next post -> " link beside "Back to Feed", pointing at the next older
  post site-wide (blocked authors skipped). Wired through the same next_url
  mechanism already used for listing pagination, so it emits a real
  backend-rendered <link rel="next"> tag for SEO, not just a visible link.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BWJy6PrMMt5hwWxQwia2rd

tests/e2e/feed.py

@@ -1,6 +1,6 @@
 # retoor <retoor@molodetz.nl>
 
-from tests.conftest import BASE_URL, create_post_with_files, paste_image
+from tests.conftest import BASE_URL, CACHE_VERSION_PROPAGATION_SECONDS, create_post_with_files, paste_image
 import time
 import requests
 from playwright.sync_api import expect
@@ -11,6 +11,7 @@ from devplacepy.database import (
     list_custom_overrides,
     set_custom_override,
     set_customization_pref,
+    set_setting,
 )
 from devplacepy.utils import clear_user_cache
 def _user_customization_toggle(username):
@@ -851,8 +852,14 @@ def test_delete_own_post(alice):
     page.locator(".post-action-btn:has-text('Delete')").click()
     page.locator(".dialog-overlay.visible .dialog-confirm").click()
     page.wait_for_url(f"{BASE_URL}/feed", wait_until="domcontentloaded")
-    resp = page.goto(post_url, wait_until="domcontentloaded")
-    assert resp.status == 404
+    set_setting("happy_404_enabled", "0")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+    try:
+        resp = page.goto(post_url, wait_until="domcontentloaded")
+        assert resp.status == 404
+    finally:
+        set_setting("happy_404_enabled", "1")
+        time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
 
 
 def test_profile_nav_from_topbar(alice):