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/tools/deepsearch/export.py

@@ -1,9 +1,10 @@
 # retoor <retoor@molodetz.nl>
 
 import json
+import time
 import pytest
 import requests
-from tests.conftest import BASE_URL
+from tests.conftest import BASE_URL, CACHE_VERSION_PROPAGATION_SECONDS
 from devplacepy.database import create_deepsearch_session, get_table, refresh_snapshot, set_setting
 from devplacepy.services.jobs import queue
 
@@ -11,7 +12,11 @@ from devplacepy.services.jobs import queue
 @pytest.fixture(scope="module", autouse=True)
 def _settings(app_server):
     set_setting("rate_limit_per_minute", "1000000")
+    set_setting("happy_404_enabled", "0")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
     yield
+    set_setting("happy_404_enabled", "1")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
 
 
 def _weasyprint_available() -> bool:

tests/api/tools/deepsearch/session.py

@@ -1,17 +1,29 @@
 # retoor <retoor@molodetz.nl>
 
 import json
+import time
 
+import pytest
 import requests
-from tests.conftest import BASE_URL
+from tests.conftest import BASE_URL, CACHE_VERSION_PROPAGATION_SECONDS
 from devplacepy.database import (
     create_deepsearch_session,
     get_table,
     refresh_snapshot,
+    set_setting,
 )
 from devplacepy.services.jobs import queue
 
 
+@pytest.fixture
+def _no_happy_404(app_server):
+    set_setting("happy_404_enabled", "0")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+    yield
+    set_setting("happy_404_enabled", "1")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+
+
 def _json_headers():
     return {"Accept": "application/json"}
 
@@ -204,6 +216,6 @@ def test_export_json(app_server):
         _clear()
 
 
-def test_export_unknown_uid_404(app_server):
+def test_export_unknown_uid_404(app_server, _no_happy_404):
     r = requests.get(f"{BASE_URL}/tools/deepsearch/nope/export.md")
     assert r.status_code == 404

tests/api/tools/isslop.py

@@ -3,16 +3,26 @@
 import time
 import uuid
 
+import pytest
 import requests
 
-from tests.conftest import BASE_URL
-from devplacepy.database import get_table, refresh_snapshot
+from tests.conftest import BASE_URL, CACHE_VERSION_PROPAGATION_SECONDS
+from devplacepy.database import get_table, refresh_snapshot, set_setting
 from devplacepy.services.jobs.isslop import store
 from devplacepy.utils import generate_uid
 
 _counter_isslop = [0]
 
 
+@pytest.fixture
+def _no_happy_404(app_server):
+    set_setting("happy_404_enabled", "0")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+    yield
+    set_setting("happy_404_enabled", "1")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+
+
 def _json_headers():
     return {"Accept": "application/json"}
 
@@ -132,7 +142,7 @@ def test_status_unknown_uid_404(app_server):
     assert r.status_code == 404
 
 
-def test_report_json_while_pending(app_server):
+def test_report_json_while_pending(app_server, _no_happy_404):
     session = requests.Session()
     try:
         run = session.post(
@@ -287,7 +297,7 @@ def test_media_route_serves_a_thumbnail(app_server):
         store.purge_analysis(uid)
 
 
-def test_media_route_rejects_a_name_outside_the_hex_pattern(app_server):
+def test_media_route_rejects_a_name_outside_the_hex_pattern(app_server, _no_happy_404):
     uid = generate_uid()
     store.create_analysis(uid, "https://github.com/owner/repository", "guest", "media-traversal-owner")
     try:

tests/api/tools/seo.py

@@ -1,11 +1,23 @@
 # retoor <retoor@molodetz.nl>
 
+import time
+
+import pytest
 import requests
-from tests.conftest import BASE_URL
-from devplacepy.database import get_table, refresh_snapshot
+from tests.conftest import BASE_URL, CACHE_VERSION_PROPAGATION_SECONDS
+from devplacepy.database import get_table, refresh_snapshot, set_setting
 from devplacepy.services.jobs import queue
 
 
+@pytest.fixture(scope="module", autouse=True)
+def _disable_happy_404(app_server):
+    set_setting("happy_404_enabled", "0")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+    yield
+    set_setting("happy_404_enabled", "1")
+    time.sleep(CACHE_VERSION_PROPAGATION_SECONDS)
+
+
 def _json_headers():
     return {"Accept": "application/json"}