fix(jira): use model_validator(after) for jira_token_set + timeout on test

FastAPI uses __pydantic_validator__.validate_python() which bypasses
model_validate() overrides. Switch to @model_validator(mode='after')
which the Pydantic Rust core always calls, so jira_token_set is now
correctly derived from the excluded jira_api_token field.

Also add a 10s timeout to the jira-test endpoint and better error
messages (the Atlassian library's "Expecting value" JSON error was
ambiguous).

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

backend/app/routers/system.py

@@ -295,7 +295,8 @@ def test_jira_connection(
 
     try:
         jira = get_user_jira_client(current_user, db)
-        # Lightweight call: get current user info
+        # Lightweight call: get current user info (10 s hard timeout)
+        jira._session.timeout = 10  # type: ignore[attr-defined]
         myself = jira.myself()
         return {
             "status": "ok",
@@ -303,10 +304,22 @@ def test_jira_connection(
             "jira_url": jira_url,
         }
     except Exception as exc:
-        raise HTTPException(
-            status_code=502,
-            detail=f"Jira connection failed: {exc}",
-        )
+        err = str(exc)
+        # Translate common Atlassian-library errors into human-readable messages
+        if "Expecting value" in err or "line 1 column 1" in err:
+            detail = (
+                "Jira returned an unexpected response — check that the URL is correct "
+                "and that the account email + API token are valid."
+            )
+        elif "401" in err or "Unauthorized" in err:
+            detail = "Authentication failed (401). Verify your Atlassian email and API token."
+        elif "403" in err or "Forbidden" in err:
+            detail = "Access denied (403). The token may not have permission to access this Jira instance."
+        elif "timed out" in err.lower() or "timeout" in err.lower():
+            detail = "Connection timed out. Check the Jira URL is reachable from the server."
+        else:
+            detail = f"Jira connection failed: {err}"
+        raise HTTPException(status_code=502, detail=detail)
 
 
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