Make L3VPN RD/VRF filter dynamic (roadmap E4)

Both L3VPN dashboards had a static custom 'rd' variable holding only
the '-' (all) sentinel — you could not actually filter by a VRF.
Convert 'rd' to a query variable that discovers route distinguishers
from l3vpn_rib. Degrades cleanly on the (currently empty) lab table:
the query always returns '-', so behaviour is unchanged until real
L3VPN data exists, then RDs auto-populate. Existing panel SQL
('$rd' = '-' OR rd = '$rd') is untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

obmp-grafana/dashboards/obmp/L3VPN-1005/l3vpn_prefix_hist.json

@@ -645,27 +645,28 @@
         "type": "query"
       },
       {
-        "current": {
-          "selected": false,
-          "text": "-",
-          "value": "-"
-        },
-        "description": "RD in the format of N:N.  Set to - for all.",
-        "hide": 0,
-        "includeAll": false,
-        "label": "RD",
-        "multi": false,
         "name": "rd",
-        "options": [
+        "type": "query",
-          {
+        "label": "RD",
+        "description": "Route Distinguisher (VRF). Auto-discovered from l3vpn_rib; '-' = all VRFs.",
+        "datasource": {
+          "type": "postgres",
+          "uid": "obmp_postgres"
+        },
+        "query": "SELECT '-' AS rd UNION SELECT DISTINCT rd FROM l3vpn_rib WHERE iswithdrawn = false ORDER BY rd",
+        "definition": "SELECT '-' AS rd UNION SELECT DISTINCT rd FROM l3vpn_rib WHERE iswithdrawn = false ORDER BY rd",
+        "refresh": 1,
+        "includeAll": false,
+        "multi": false,
+        "current": {
           "selected": true,
           "text": "-",
           "value": "-"
-          }
+        },
-        ],
+        "options": [],
-        "query": "-",
+        "sort": 1,
-        "skipUrlSync": false,
+        "hide": 0,
-        "type": "custom"
+        "skipUrlSync": false
       }
     ]
   },

obmp-grafana/dashboards/obmp/L3VPN-1005/l3vpn_rib_browser.json

@@ -1067,28 +1067,28 @@
         "useTags": false
       },
       {
+        "name": "rd",
+        "type": "query",
+        "label": "RD",
+        "description": "Route Distinguisher (VRF). Auto-discovered from l3vpn_rib; '-' = all VRFs.",
+        "datasource": {
+          "type": "postgres",
+          "uid": "obmp_postgres"
+        },
+        "query": "SELECT '-' AS rd UNION SELECT DISTINCT rd FROM l3vpn_rib WHERE iswithdrawn = false ORDER BY rd",
+        "definition": "SELECT '-' AS rd UNION SELECT DISTINCT rd FROM l3vpn_rib WHERE iswithdrawn = false ORDER BY rd",
+        "refresh": 1,
+        "includeAll": false,
+        "multi": false,
         "current": {
           "selected": true,
           "text": "-",
           "value": "-"
         },
-        "description": "RD in the format of N:N.  Set to - for all.",
+        "options": [],
+        "sort": 1,
         "hide": 0,
-        "includeAll": false,
+        "skipUrlSync": false
-        "label": "RD",
-        "multi": false,
-        "name": "rd",
-        "options": [
-          {
-            "selected": true,
-            "text": "-",
-            "value": "-"
-          }
-        ],
-        "query": "-",
-        "queryValue": "203:20",
-        "skipUrlSync": false,
-        "type": "custom"
       },
       {
         "current": {