feat(syslog): a device through Docker's address is placed by its name, or a device address set on it (#6096)

The SNMP poll asks each device's sysName in the same GET and keeps it
(snmp_003_sys_name). A syslog source told apart by name finds its own
address from the SNMP device (reported or given name) or host of that
name, so its lines reach the SNMP device's and host's pages again; the
edit page gains Device address, which always wins (syslog_004).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_snmp_poll.py

@@ -48,8 +48,9 @@ def test_a_poll_records_why_a_device_is_silent_and_what_another_answered(app):
             return PollResult(error="No reply from 10.9.9.9:161 within 5 s.")
         keys = dict((k, o) for o, k in oids)
         assert {"uptime", "if1_in", "if1_status"} <= set(keys) and creds.community == "public"
+        assert keys["_sys_name"] == "1.3.6.1.2.1.1.5.0"                  # the device's own name rides along
         return PollResult(readings={"uptime": 8_640_000.0, "if1_status": 1.0, "if1_in": next(counts)},
-                          missing=["if1_out"])
+                          texts={"_sys_name": "SW-ONE"}, missing=["if1_out"])
 
     async def go():
         from steward.core.crypto import encrypt_secret
@@ -79,6 +80,8 @@ def test_a_poll_records_why_a_device_is_silent_and_what_another_answered(app):
 
     first, second, rows = asyncio.run(go())
     assert rows[up].last_error is None and rows[up].last_ok_at is not None and rows[up].missing == ["if1_out"]
+    assert rows[up].sys_name == "SW-ONE" and rows[down].sys_name == ""
+    assert "_sys_name" not in (rows[up].texts or {})
     assert rows[down].last_error == "No reply from 10.9.9.9:161 within 5 s." and rows[down].last_ok_at is None
     assert rows[down].last_polled_at is not None
     assert first == {"uptime": 8_640_000.0, "if1_status": 1.0}          # a counter needs two samples