monitoring(wealth): monthly contributions vs market gain bar chart

Goal stated by user: see when monthly market gain starts to exceed
monthly contributions, i.e. the inflection point where the market is
out-earning savings rather than the other way around.

New panel id=25 between the annual decomposition (13) and per-account
ROI (14): bar chart with two side-by-side bars per month --
contributions (blue) and market gain (green). Same calculation as
panel 13 but month-grain instead of year-grain. Months where the
green bar dwarfs the blue one are visible at a glance.

SQL: same endpoints CTE pattern as panel 13, with date_trunc('month',
valuation_date) as the grouping key. Uses max_complete cutoff so
partial-today doesn't skew the latest month.

Layout: panels at y >= 75 shifted down by 11 (chart height). New
chart at y=75; panel 14 (per-account ROI) -> y=86; panel 10
(activity log) -> y=96.

Spot check (recent months from PG):
  2025-07: contrib +£5,601    market +£42,295   <- big market month
  2025-09: contrib +£1,501    market +£24,206
  2026-02: contrib +£35,501   market +£41,382
  2026-03: contrib +£5,501    market -£38,483   <- correction
  2026-04: contrib +£73,267   market +£21,448

stacks/monitoring/modules/monitoring/dashboards/wealth.json

@@ -728,7 +728,7 @@
         "h": 14,
         "w": 24,
         "x": 0,
-        "y": 85
+        "y": 96
       },
       "fieldConfig": {
         "defaults": {
@@ -1702,6 +1702,124 @@
         }
       ]
     },
+    {
+      "id": 25,
+      "title": "Monthly contributions vs market gain",
+      "description": "Each month split into 'new money in' (contributions) and 'market gain' (everything else). Side-by-side bars, so months where market gain exceeds contributions are visually obvious — that's the inflection point where your investments out-earn your savings.",
+      "type": "barchart",
+      "datasource": {
+        "type": "grafana-postgresql-datasource",
+        "uid": "wealth-pg"
+      },
+      "gridPos": {
+        "h": 11,
+        "w": 24,
+        "x": 0,
+        "y": 75
+      },
+      "fieldConfig": {
+        "defaults": {
+          "color": {
+            "mode": "palette-classic"
+          },
+          "unit": "currencyGBP",
+          "decimals": 0,
+          "custom": {
+            "axisPlacement": "auto",
+            "axisLabel": "",
+            "fillOpacity": 80,
+            "gradientMode": "none",
+            "lineWidth": 1
+          }
+        },
+        "overrides": [
+          {
+            "matcher": {
+              "id": "byName",
+              "options": "month"
+            },
+            "properties": [
+              {
+                "id": "unit",
+                "value": "string"
+              }
+            ]
+          },
+          {
+            "matcher": {
+              "id": "byName",
+              "options": "contributions"
+            },
+            "properties": [
+              {
+                "id": "color",
+                "value": {
+                  "mode": "fixed",
+                  "fixedColor": "blue"
+                }
+              },
+              {
+                "id": "displayName",
+                "value": "Net contributions"
+              }
+            ]
+          },
+          {
+            "matcher": {
+              "id": "byName",
+              "options": "market_gain"
+            },
+            "properties": [
+              {
+                "id": "color",
+                "value": {
+                  "mode": "fixed",
+                  "fixedColor": "#56A64B"
+                }
+              },
+              {
+                "id": "displayName",
+                "value": "Market gain"
+              }
+            ]
+          }
+        ]
+      },
+      "options": {
+        "barRadius": 0,
+        "barWidth": 0.7,
+        "groupWidth": 0.75,
+        "orientation": "auto",
+        "showValue": "auto",
+        "stacking": "none",
+        "xField": "month",
+        "xTickLabelRotation": -45,
+        "legend": {
+          "calcs": [
+            "sum"
+          ],
+          "displayMode": "table",
+          "placement": "bottom"
+        },
+        "tooltip": {
+          "mode": "multi",
+          "sort": "desc"
+        }
+      },
+      "targets": [
+        {
+          "refId": "A",
+          "datasource": {
+            "type": "grafana-postgresql-datasource",
+            "uid": "wealth-pg"
+          },
+          "rawQuery": true,
+          "editorMode": "code",
+          "format": "table",
+          "rawSql": "WITH active_count AS (SELECT COUNT(*) AS n FROM accounts), max_complete AS (SELECT MAX(valuation_date) AS d FROM (SELECT d.valuation_date, COUNT(*) AS c FROM daily_account_valuation d JOIN accounts a ON a.id = d.account_id GROUP BY d.valuation_date) x WHERE c >= (SELECT n FROM active_count)), monthly AS (SELECT date_trunc('month', valuation_date)::date AS month, valuation_date, SUM(total_value) AS nw, SUM(net_contribution) AS contrib FROM daily_account_valuation WHERE valuation_date <= (SELECT d FROM max_complete) GROUP BY valuation_date), endpoints AS (SELECT month, (array_agg(nw ORDER BY valuation_date ASC))[1] AS nw_start, (array_agg(nw ORDER BY valuation_date DESC))[1] AS nw_end, (array_agg(contrib ORDER BY valuation_date ASC))[1] AS contrib_start, (array_agg(contrib ORDER BY valuation_date DESC))[1] AS contrib_end FROM monthly GROUP BY month) SELECT month::text AS month, ROUND((contrib_end - contrib_start)::numeric, 0) AS contributions, ROUND((nw_end - nw_start - (contrib_end - contrib_start))::numeric, 0) AS market_gain FROM endpoints ORDER BY month"
+        }
+      ]
+    },
     {
       "id": 14,
       "title": "Per-account ROI %",
@@ -1715,7 +1833,7 @@
         "h": 10,
         "w": 24,
         "x": 0,
-        "y": 75
+        "y": 86
       },
       "fieldConfig": {
         "defaults": {