feat(marker): add project-level marker definition inheritance

Project-level marker definitions fan out to every link (existing and new).
A definition is stored once on Project._marker_definitions; when applied to
a link the marker is named "global-{def_name}" — the "global" prefix was
pre-reserved in the schema, so inherited and per-link markers can never
collide, nor will their registry keys.

Key behavior:
- POST /projects/{pid}/marker-definitions → fan out to all existing links
- PUT  /projects/{pid}/marker-definitions/{name} → sync all inherited copies
- DELETE → remove every inherited copy from every link
- New links auto-inherit all active defs (hook in UDPLink.create)
- Per-link DELETE/PUT of a "global-*" marker is rejected (409)
- Inherited markers are NOT persisted in the topology; they are re-created
  from _marker_definitions on project load
- Compute side is untouched — the marker reaches uBridge via the existing
  start_marker→update→NIO→ubridge pipeline

Files:
- controller/project.py — _marker_definitions + CRUD + fanout + topology load
- controller/link.py — Link.inherit_marker() + asdict() filter
- controller/udp_link.py — guards on stop/update + create() inheritance hook
- controller/topology.py — persist marker_definitions in project topology
- schemas/controller/links.py — MarkerDefinitionCreate schema
- api/routes/controller/projects.py — REST endpoints (marker-definitions)

gns3server/controller/link.py

@@ -107,6 +107,32 @@ class Link:
         """
         return self._markers
 
+    async def inherit_marker(self, def_name, marker_def):
+        """
+        Apply a project-level marker definition to this link.
+
+        The marker is stored under ``global-{def_name}`` so it cannot collide
+        with a per-link private marker of the same name.  It carries an
+        ``inherited_from`` back-reference that (a) guards against per-link
+        edits and (b) lets the project sync changes to every copy at once.
+        """
+
+        await self.start_marker(
+            name=f"global-{def_name}",
+            bpf=marker_def["bpf"],
+            tag=marker_def.get("tag"),
+            color=marker_def.get("color"),
+            inherited_from=def_name,
+        )
+
+    def _persist_markers(self):
+        """
+        Return only the per-link (non-inherited) markers suitable for
+        persistence in a topology dump.  Inherited markers are re-created from
+        ``project._marker_definitions`` on load so they do not need to be saved.
+        """
+        return {k: v for k, v in self._markers.items() if not v.get("inherited_from")}
+
     @property
     def show_filters_icon(self):
         """
@@ -600,7 +626,7 @@ class Link:
                 "nodes": res,
                 "link_id": self._id,
                 "filters": self._filters,
-                "markers": self._markers,
+                "markers": self._persist_markers(),
                 "link_style": self._link_style,
                 "suspend": self._suspended,
                 "show_filters_icon": getattr(self, '_show_filters_icon', True),
@@ -615,7 +641,7 @@ class Link:
             "capture_compute_id": self.capture_compute_id,
             "link_type": self._link_type,
             "filters": self._filters,
-            "markers": self._markers,
+            "markers": self._persist_markers(),
             "suspend": self._suspended,
             "link_style": self._link_style,
             "wireshark": self._wireshark,