fix: fetch remote-compute marker pcaps in tag replay

Marker pcaps are written by uBridge on the compute that hosts the capture
node. The tag replay only looked in the project folder on the controller.
So a source from a remote compute showed count 0 and no frames, with no
error. Its frame detail returned 404 and said nothing ever matched (#3010).

Each source now gets its pcap through the compute of its capture node.
Local files are read as before. Remote files are fetched over HTTP on
every acquire into a small cache (32 idle files, cleared at shutdown).
A file is replaced only when its bytes changed. This keeps the old mtime,
so the sharkd session for that file stays alive.

When a compute cannot be reached, its source now shows an error message
in the timeline instead of just showing nothing. The detail endpoint now
returns an error instead of 404.

gns3server/schemas/controller/marker_replay.py

@@ -29,6 +29,9 @@ class ReplaySource(BaseModel):
     marker: str
     data_link_type: str
     count: int = Field(..., description="Total frames in the capture, unaffected by filters")
+    error: Optional[str] = Field(
+        None, description="Set when the capture file could not be fetched from the compute hosting it"
+    )
 
 
 class ReplayFrame(BaseModel):