Implement flexible entity relation system (Phases A–E)

Declarative relation registry via defrelation s-expressions with
cardinality enforcement (one-to-one, one-to-many, many-to-many),
registry-aware relate/unrelate/can-relate API endpoints, generic
container-nav fragment, and relation-driven UI components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

relations/bp/fragments/routes.py

@@ -0,0 +1,88 @@
+"""Relations app fragment endpoints.
+
+Generic container-nav fragment that renders navigation items for all
+related entities, driven by the relation registry.
+"""
+
+from __future__ import annotations
+
+from quart import Blueprint, Response, g, request
+
+from shared.infrastructure.fragments import FRAGMENT_HEADER
+
+
+def register():
+    bp = Blueprint("fragments", __name__, url_prefix="/internal/fragments")
+
+    _handlers: dict[str, object] = {}
+
+    @bp.before_request
+    async def _require_fragment_header():
+        if not request.headers.get(FRAGMENT_HEADER):
+            return Response("", status=403)
+
+    @bp.get("/<fragment_type>")
+    async def get_fragment(fragment_type: str):
+        handler = _handlers.get(fragment_type)
+        if handler is None:
+            return Response("", status=200, content_type="text/html")
+        html = await handler()
+        return Response(html, status=200, content_type="text/html")
+
+    # --- generic container-nav fragment ----------------------------------------
+
+    async def _container_nav_handler():
+        """Render nav items for all visible relations of a container entity.
+
+        Query params:
+            container_type: entity type (e.g. "page")
+            container_id:   entity id
+            post_slug:      used for URL construction
+            exclude:        comma-separated relation types to skip
+        """
+        from shared.sexp.jinja_bridge import sexp as render_sexp
+        from shared.sexp.relations import relations_from
+        from shared.services.relationships import get_children
+
+        container_type = request.args.get("container_type", "page")
+        container_id = int(request.args.get("container_id", 0))
+        post_slug = request.args.get("post_slug", "")
+        exclude_raw = request.args.get("exclude", "")
+        exclude = set(exclude_raw.split(",")) if exclude_raw else set()
+
+        nav_defs = [
+            d for d in relations_from(container_type)
+            if d.nav != "hidden" and d.name not in exclude
+        ]
+
+        if not nav_defs:
+            return ""
+
+        parts = []
+        for defn in nav_defs:
+            children = await get_children(
+                g.s,
+                parent_type=container_type,
+                parent_id=container_id,
+                child_type=defn.to_type,
+                relation_type=defn.name,
+            )
+            for child in children:
+                slug = (child.metadata_ or {}).get("slug", "")
+                href = f"/{post_slug}/{slug}/" if post_slug else f"/{slug}/"
+                parts.append(render_sexp(
+                    '(~relation-nav :href href :name name :icon icon :nav-class nav-class :relation-type relation-type)',
+                    href=href,
+                    name=child.label or "",
+                    icon=defn.nav_icon or "",
+                    **{
+                        "nav-class": "",
+                        "relation-type": defn.name,
+                    },
+                ))
+
+        return "\n".join(parts)
+
+    _handlers["container-nav"] = _container_nav_handler
+
+    return bp