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

README.md

@@ -288,6 +288,7 @@ Operational behavior is tunable live from `/admin/settings` (stored in `site_set
 | `registration_open` | `1` | When `0`, new sign-ups are rejected |
 | `maintenance_mode` | `0` | When `1`, non-admins see the maintenance page; admins retain access |
 | `maintenance_message` | scheduled-maintenance text | Message shown during maintenance |
+| `happy_404_enabled` | `1` | When `1`, an HTML page that would otherwise 404 renders a random existing post instead ("Happy 404"); JSON/API requests always get a real 404 regardless. See [Happy 404](#happy-404) |
 | `customization_enabled` | `1` | When `0`, no user CSS/JS customization is injected on any page |
 | `customization_js_enabled` | `1` | When `0`, user custom CSS is still served but custom JavaScript is suppressed |
 | `audit_log_retention_days` | `90` | Audit rows older than this are pruned daily by the Audit retention service; `0` disables pruning |
@@ -342,6 +343,27 @@ curl -H "Accept: application/json" https://your-host/feed
 curl -H "Accept: application/json" -X POST -d "content=hi&title=T&topic=devlog" https://your-host/posts/create
 ```
 
+## Happy 404
+
+Instead of a bare error page, an HTML request that would 404 (an unmatched route, or an app route
+raising `not_found()` for a missing resource) instead renders a random existing post at that URL,
+using the exact same template and context as the real `/posts/{slug}` page. JSON/API requests are
+unaffected and still get a normal `404` - the substitution only ever applies to a browser HTML
+navigation.
+
+- **Toggle:** `happy_404_enabled` site setting (`/admin/settings`, on by default).
+- **Scope:** any HTML `GET` 404, app-wide - not just under `/posts`.
+- **Performance:** a small pool of random post slugs is cached in-process for a few minutes and
+  refreshed with a fresh random sample on expiry, so every request only does an in-memory pick plus
+  one indexed lookup - no per-request full-table scan - while the pool composition still cycles
+  through the whole `posts` table over time.
+- **SEO safety:** the substituted page is always marked `noindex,nofollow` so the decoy URL is never
+  indexed under the wrong address.
+- **Fail-closed:** any error while building the substitute page falls straight through to the normal
+  404 page - this feature can never turn a real error into a worse one.
+
+Implementation: `devplacepy/happy404.py`, wired into the `404` exception handler in `main.py`.
+
 ## XML-RPC bridge
 
 The full REST API is also reachable over XML-RPC at `/xmlrpc`. A standalone forking XML-RPC