fix(notifications): scope routing rules to nodes via node_id column (#775)

Adds a nullable node_id column to notification_routes (null = any
node, integer = fire only when the alert originates from that specific
node). This fixes a multi-node fleet defect where a route scoped to
"my-app" would fire on every node that hosts a stack with that name.

Backend changes:
- DatabaseService: idempotent migration adds node_id INTEGER NULL and
  a composite index on (node_id, enabled, priority); the two statements
  are in separate try-catch blocks so the index is always created even
  when the column was added in an earlier run
- NotificationService: route matcher now pre-filters by node_id before
  checking stack_patterns (== null matches any node)
- notifications route: POST/PUT accept optional node_id, validated to
  be null or the local node's ID; NodeRegistry guards against
  cross-node misroutes

Frontend changes:
- NotificationRoutingSection: node scope Select field uses useNodes()
  from NodeContext (no extra API call) to populate the local node option
- Route cards show a node badge when node_id is set

Tests: 3 new tests covering node-match, node-mismatch, and null-scope;
all 75 files (1413 tests) passing.

backend/src/services/NotificationService.ts

@@ -122,7 +122,10 @@ export class NotificationService {
 
         if (stackName !== undefined) {
             const routes = this.dbService.getEnabledNotificationRoutes();
-            const matched = routes.filter(r => r.stack_patterns.includes(stackName));
+            const matched = routes.filter(r =>
+                (r.node_id == null || r.node_id === localNodeId) &&
+                r.stack_patterns.includes(stackName)
+            );
             if (matched.length > 0) {
                 if (isDebugEnabled()) console.log(`[Notify:diag] Matched ${matched.length} route(s) for stack "${stackName}"`);
                 await Promise.allSettled(