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/api/error/pages.py

@@ -1,10 +1,80 @@
 # retoor <retoor@molodetz.nl>
 
+import time
+
 import requests
-from tests.conftest import BASE_URL
+
+from devplacepy.database import set_setting
+from tests.conftest import BASE_URL, CACHE_VERSION_PROPAGATION_SECONDS
+
+JSON = {"Accept": "application/json"}
+
+
+def _set_happy_404(enabled: bool) -> None:
+    set_setting("happy_404_enabled", "1" if enabled else "0")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+
+
+def _login(seeded_db, who="alice"):
+    creds = seeded_db[who]
+    session = requests.Session()
+    session.post(
+        f"{BASE_URL}/auth/login",
+        data={"email": creds["email"], "password": creds["password"]},
+    )
+    return session
 
 
 def test_404_renders_error_page(app_server):
-    r = requests.get(f"{BASE_URL}/this-page-does-not-exist-xyz")
+    _set_happy_404(False)
+    try:
+        r = requests.get(f"{BASE_URL}/this-page-does-not-exist-xyz")
+        assert r.status_code == 404
+        assert "404" in r.text or "not found" in r.text.lower()
+    finally:
+        _set_happy_404(True)
+
+
+def test_happy_404_renders_a_real_post(app_server, seeded_db):
+    session = _login(seeded_db)
+    title = f"Happy404 Marker {int(time.time() * 1000)}"
+    created = session.post(
+        f"{BASE_URL}/posts/create",
+        headers=JSON,
+        data={
+            "title": title,
+            "content": "content seeded so the happy 404 pool has something to pick.",
+            "topic": "devlog",
+        },
+    ).json()["data"]
+    assert created["uid"]
+
+    _set_happy_404(True)
+    r = requests.get(f"{BASE_URL}/this-path-was-never-registered-anywhere")
+    assert r.status_code == 200
+    assert 'class="post-detail"' in r.text
+    assert "Page not found" not in r.text
+    assert 'name="robots" content="noindex,nofollow"' in r.text
+
+
+def test_happy_404_disabled_falls_back_to_error_page(app_server):
+    _set_happy_404(False)
+    try:
+        r = requests.get(f"{BASE_URL}/another-path-that-does-not-exist")
+        assert r.status_code == 404
+        assert "Page not found" in r.text
+    finally:
+        _set_happy_404(True)
+
+
+def test_happy_404_never_applies_to_json_requests():
+    _set_happy_404(True)
+    r = requests.get(f"{BASE_URL}/yet-another-missing-path", headers=JSON)
+    assert r.status_code == 404
+    assert r.json()["error"]["status"] == 404
+
+
+def test_happy_404_never_applies_to_api_paths():
+    _set_happy_404(True)
+    r = requests.get(f"{BASE_URL}/api/this-devrant-route-does-not-exist")
     assert r.status_code == 404
-    assert "404" in r.text or "not found" in r.text.lower()