Count hand-valued assets into the wealth figure

A wealth figure that ignores the house is not a wealth figure. Assets
without a market feed - a house, a car, a private loan - are now added
by hand on the Wealth page with a stated value, a currency and the day
the estimate was made; a negative value records a liability. They are
registry entities in assets.finance like everything else, join the
per-currency totals immediately, and a currency held only in an asset
earns its own line.

internal/domain/model.go

@@ -116,6 +116,20 @@ type Instrument struct {
 	QuotedAt string   `json:"quoted_at,omitempty"`
 }
 
+// Asset is a possession valued by hand: a house, a car, anything without a
+// market feed. Value is what the owner states it is worth and ValuedAt the day
+// that estimate was made, so a stale figure is visible rather than silently
+// trusted. A negative value records a liability such as a mortgage.
+type Asset struct {
+	ID   string `json:"id"`
+	Name string `json:"name"`
+	// Kind is free display text grouping the asset: "Real estate", "Vehicle".
+	Kind     string `json:"kind,omitempty"`
+	Currency string `json:"currency"`
+	Value    Money  `json:"value"`
+	ValuedAt string `json:"valued_at"`
+}
+
 type Facts struct {
 	ID               string `json:"id"`
 	Source           string `json:"source"`
@@ -178,6 +192,7 @@ type Dataset struct {
 	Tags         []Tag         `json:"tags"`
 	Merchants    []Merchant    `json:"merchants"`
 	Instruments  []Instrument  `json:"instruments"`
+	Assets       []Asset       `json:"assets"`
 	Transactions []Transaction `json:"transactions"`
 }