POST /api/model-endpoints always inserted a new row, so Settings -> Add
Models -> Scan for Servers re-added any endpoint a user had already
registered manually — once under its model name (from the earlier
manual add) and again under its host:port (auto-generated when scan
posts without a name). The success toast then misreported the result
as "added N new".
Look up an existing endpoint with the same base_url accessible to the
caller (shared or owned by them) before inserting. If found, return it
with `existing: true` so the client can tell the difference between
an actual add and a dedupe hit. Toast now reads, e.g.,
"Found 1 server with 1 model — 1 already added".
Tested: POSTing the same base_url three times (incl. trailing-slash
variation) returns the same id each time; only one row exists.
