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

devplacepy/database/schema.py

@@ -1964,6 +1964,7 @@ def init_db():
             "maintenance_message": "DevPlace is undergoing scheduled maintenance. Please check back shortly.",
             "customization_enabled": "1",
             "customization_js_enabled": "1",
+            "happy_404_enabled": "1",
             "audit_log_retention_days": "90",
             "statistics_tracking_enabled": "1",
             "docs_search_mode": "agent",