feat: watcher agent management system implemented.

backend/models.py

@@ -66,6 +66,26 @@ class RosterUnit(Base):
     slm_last_check = Column(DateTime, nullable=True)  # Last communication check
 
 
+class WatcherAgent(Base):
+    """
+    Watcher agents: tracks the watcher processes (series3-watcher, thor-watcher)
+    that run on field machines and report unit heartbeats.
+
+    Updated on every heartbeat received from each source_id.
+    """
+    __tablename__ = "watcher_agents"
+
+    id = Column(String, primary_key=True, index=True)        # source_id (hostname)
+    source_type = Column(String, nullable=False)              # series3_watcher | series4_watcher
+    version = Column(String, nullable=True)                   # e.g. "1.4.0"
+    last_seen = Column(DateTime, default=datetime.utcnow)
+    status = Column(String, nullable=False, default="unknown") # ok | pending | missing | error | unknown
+    ip_address = Column(String, nullable=True)
+    log_tail = Column(Text, nullable=True)                    # last N log lines (JSON array of strings)
+    update_pending = Column(Boolean, default=False)           # set True to trigger remote update
+    update_version = Column(String, nullable=True)            # target version to update to
+
+
 class IgnoredUnit(Base):
     """
     Ignored units: units that report but should be filtered out from unknown emitters.