feat: improve network topology — LLDP-authoritative, manual links, orphan handling (v0.11.6 Beta)

- Backend: 7-step pipeline — per-neighbor best-protocol dedup, bidirectional
  LLDP pair merging, LLDP-covered set suppression of noisy CDP/MNDP links,
  shared-segment heuristic limited to genuinely unresolved non-LLDP links
- Backend: manual_topology_links table + POST/DELETE endpoints for user-drawn connections
- Frontend: ManualEdge custom edge type (purple dashed + × delete button)
- Frontend: Connect Mode toggle — blue crosshair handles, onConnect creates DB-persisted link
- Frontend: orphan nodes in dedicated bottom row with orange warning banner
- Frontend: legend updated, canvas height increased

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

backend/src/db/migrate.ts

@@ -347,6 +347,18 @@ CREATE TABLE IF NOT EXISTS device_availability (
 );
 CREATE INDEX IF NOT EXISTS idx_device_availability_device ON device_availability(device_id, went_offline_at DESC);
 
+-- Manual topology links — user-drawn connections for devices with no auto-discovery
+CREATE TABLE IF NOT EXISTS manual_topology_links (
+  id               SERIAL PRIMARY KEY,
+  from_device_id   INTEGER NOT NULL REFERENCES devices(id) ON DELETE CASCADE,
+  to_device_id     INTEGER NOT NULL REFERENCES devices(id) ON DELETE CASCADE,
+  label            VARCHAR(100),
+  created_at       TIMESTAMPTZ DEFAULT NOW(),
+  UNIQUE(from_device_id, to_device_id)
+);
+CREATE INDEX IF NOT EXISTS idx_manual_topology_links_from ON manual_topology_links(from_device_id);
+CREATE INDEX IF NOT EXISTS idx_manual_topology_links_to   ON manual_topology_links(to_device_id);
+
 -- Configuration templates (reusable config sets pushed to devices or groups)
 CREATE TABLE IF NOT EXISTS config_templates (
   id               SERIAL PRIMARY KEY,