feat(syslog): devices through Swarm's ingress told apart by name; the send-to address is a setting (#6096)

Through ingress every device arrives from Docker's own address, so a
source there is keyed by the hostname its lines carry (new column
sender, unique with address). The set-up help no longer guesses the
address from the web hostname: it shows the plugin's send_to setting,
covers ingress and host mode, ASUSTOR ADM, and both syslog formats.

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

plugins/syslog/migrations/versions/syslog_003_sender.py

@@ -0,0 +1,30 @@
+"""Syslog: devices that reach Steward through Docker's one address, told apart
+by the name in their lines
+
+Revision ID: syslog_003_sender
+Revises: syslog_002_patterns
+Create Date: 2026-10-19
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "syslog_003_sender"
+down_revision: Union[str, None] = "syslog_002_patterns"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("syslog_sources",
+                  sa.Column("sender", sa.String(255), nullable=False, server_default=""))
+    op.drop_constraint("syslog_sources_address_key", "syslog_sources", type_="unique")
+    op.create_unique_constraint("uq_syslog_sources_address_sender", "syslog_sources", ["address", "sender"])
+
+
+def downgrade() -> None:
+    op.drop_constraint("uq_syslog_sources_address_sender", "syslog_sources", type_="unique")
+    op.execute("DELETE FROM syslog_sources WHERE sender <> ''")
+    op.create_unique_constraint("syslog_sources_address_key", "syslog_sources", ["address"])
+    op.drop_column("syslog_sources", "sender")