Add GET /api/currencies/:date endpoint for historical EUR/DKK rates

Returns rates for a given date: today uses latest DB row, past dates
check DB first then fetch from Frankfurter and persist for caching.
Future dates and invalid formats return 400.

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

src/clj/pmagnus/btcdata/web/routes/api.clj

@@ -2,6 +2,7 @@
   (:require
    [clojure.data.json :as json]
    [integrant.core :as ig]
+   [pmagnus.btcdata.frankfurter.rates :as rates]
    [pmagnus.btcdata.web.controllers.health :as health]
    [pmagnus.btcdata.web.middleware.exception :as exception]
    [pmagnus.btcdata.web.middleware.formats :as formats]
@@ -12,6 +13,8 @@
    [reitit.swagger :as swagger])
   (:import
    [io.undertow.websockets.core WebSockets WebSocketChannel]
+   [java.time LocalDate]
+   [java.time.format DateTimeParseException]
    [java.util.concurrent LinkedBlockingQueue TimeUnit]))
 
 (defn- price->json [{:keys [price prev-price recorded-at]} rates]
@@ -95,6 +98,51 @@
        :headers {"Content-Type" "application/json"}
        :body    (json/write-str {:error "No price data yet"})})))
 
+(defn- rate-row->json [row]
+  (json/write-str {:rate_date (str (:rate_date row))
+                   :eur       (str (:eur row))
+                   :dkk       (str (:dkk row))
+                   :eur_dkk   (str (:eur_dkk row))}))
+
+(defn- currencies-handler [{:keys [query-fn frankfurter]} req]
+  (let [date-str (get-in req [:path-params :date])]
+    (try
+      (let [date  (LocalDate/parse date-str)
+            today (LocalDate/now)]
+        (cond
+          (.isAfter date today)
+          {:status  400
+           :headers {"Content-Type" "application/json"}
+           :body    (json/write-str {:error "Future date not allowed"})}
+
+          (.isEqual date today)
+          (if-let [row (query-fn :get-latest-currency-rates {})]
+            {:status  200
+             :headers {"Content-Type" "application/json"}
+             :body    (rate-row->json row)}
+            {:status  404
+             :headers {"Content-Type" "application/json"}
+             :body    (json/write-str {:error "No rates available yet"})})
+
+          :else
+          (if-let [row (query-fn :get-currency-rates-by-date {:rate-date date})]
+            {:status  200
+             :headers {"Content-Type" "application/json"}
+             :body    (rate-row->json row)}
+            (if-let [result (rates/fetch-and-persist-for-date!
+                              (:client frankfurter) (:url frankfurter)
+                              query-fn date)]
+              {:status  200
+               :headers {"Content-Type" "application/json"}
+               :body    (rate-row->json result)}
+              {:status  502
+               :headers {"Content-Type" "application/json"}
+               :body    (json/write-str {:error "Failed to fetch rates from Frankfurter"})}))))
+      (catch DateTimeParseException _
+        {:status  400
+         :headers {"Content-Type" "application/json"}
+         :body    (json/write-str {:error "Invalid date format, use YYYY-MM-DD"})}))))
+
 (defn- api-routes [opts]
   [["/swagger.json"
     {:get {:no-doc  true
@@ -108,6 +156,8 @@
      :middleware []}]
    ["/price/latest"
     {:get (fn [req] (latest-price-handler opts req))}]
+   ["/currencies/:date"
+    {:get (fn [req] (currencies-handler opts req))}]
    ["/strike/ws"
     {:get        (fn [req] (ws-strike-handler opts req))
      :no-doc     true