fix: add /posts/{slug} detail route so post titles resolve

Post cards on the home page have linked to /posts/<slug> since
Phase 1 (per the partial's inline comment), but the matching route
and template were never registered — clicking a post title returned
a JSON 404 from FastAPI. This adds:

- PostService.get_published_by_slug() — status-filtered, parameterized
  read that treats "draft" and "unknown slug" as the same 404 so
  unpublished titles cannot be enumerated via URL guessing.
- GET /posts/{slug} public route that 404s on miss.
- public/post.html detail template mirroring about.html's safe-render
  pattern for the bleach-sanitized body_html_cached.
- Supporting .page-article__date / .page-article__back CSS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/public.py

@@ -27,7 +27,7 @@ from fastapi.responses import HTMLResponse, Response
 from fastapi.templating import Jinja2Templates
 
 from app.config import Settings, get_settings
-from app.models.entities import Page
+from app.models.entities import Page, Post
 from app.models.posts import PostSummary
 from app.services.contact import ContactService
 from app.services.hcaptcha import HCaptchaService
@@ -117,6 +117,33 @@ def home(
     )
 
 
+@router.get(
+    "/posts/{slug}",
+    response_class=HTMLResponse,
+    summary="Single blog post",
+)
+def post_detail(
+    slug: str,
+    request: Request,
+    templates: Jinja2Templates = Depends(get_templates),
+    posts: PostService = Depends(get_post_service),
+) -> HTMLResponse:
+    """Render a single published post by slug.
+
+    Drafts and unknown slugs return 404 (same response so a mistyped
+    URL cannot be used to enumerate unpublished titles).
+    """
+    post: Post | None = posts.get_published_by_slug(slug)
+    if post is None:
+        raise HTTPException(status_code=404, detail="Post not found")
+
+    return templates.TemplateResponse(
+        request,
+        "public/post.html",
+        {"active_nav": "home", "post": post},
+    )
+
+
 @router.get("/about", response_class=HTMLResponse, summary="About the farm")
 def about(
     request: Request,