feat(api): Add Swagger UI for v1 endpoints

Serve interactive docs at /api, covering only the current /api/v1/*
endpoints. Deprecated /api/diyanet/* endpoints are left undocumented.

internal/api/docs.go

@@ -0,0 +1,27 @@
+package api
+
+import _ "embed"
+
+//go:embed openapi.yaml
+var openapiSpec []byte
+
+const swaggerHTML = `<!doctype html>
+<html>
+<head>
+    <meta charset="UTF-8">
+    <title>Prayer Times API</title>
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/swagger-ui-dist@5.33.1/swagger-ui.css">
+</head>
+<body>
+<div id="swagger-ui"></div>
+<script src="https://cdn.jsdelivr.net/npm/swagger-ui-dist@5.33.1/swagger-ui-bundle.js"></script>
+<script>
+    window.onload = () => {
+        SwaggerUIBundle({
+            url: "/api/openapi.yaml",
+            dom_id: "#swagger-ui",
+        });
+    };
+</script>
+</body>
+</html>`