Kiosk: schedule client (conditional GET + atomic cache) and engine

Adds api_client support for GET /licenses/{key}/schedule with If-None-Match
and target-payload parsing (version/type/daily/weekly + legacy blocks), an
atomic on-disk schedule cache, a pure schedule engine (half-open blocks,
midnight self-wrap for daily, Sunday->Monday spillover for weekly, invalid
blocks skipped with a warning), and a background ScheduleManager that loads
the cache offline and pulls on boot and every 5 minutes. Adds engine/cache
tests against the shared docs/fixtures, including 304, corrupt cache, and
offline cases.

src/network/api_client.h

@@ -23,6 +23,7 @@ constexpr const char* PATH_SCHEDULE = "/schedule";
 struct HttpResponse {
     long status = 0;         // HTTP status code; 0 if the request never completed
     std::string body;        // raw response body (any status)
+    std::string etag;        // response ETag header, if any
     bool transportOk = false; // true when the request completed without a transport error
 };
 
@@ -31,6 +32,9 @@ HttpResponse httpGet(const std::string& path);
 HttpResponse httpPost(const std::string& path, const std::string& jsonBody);
 HttpResponse httpPut(const std::string& path, const std::string& jsonBody);
 
+// GET with an optional If-None-Match header. Captures the response ETag.
+HttpResponse httpGetConditional(const std::string& path, const std::string& ifNoneMatch);
+
 // ─── GET /frequencies ──────────────────────────────────────────────────
 
 struct Frequency {
@@ -176,4 +180,54 @@ bool parseScheduleJson(const std::string& body, ScheduleResponse& out);
 bool fetchSchedule(const std::string& email, const std::string& license_key,
                    ScheduleResponse& out);
 
+// ─── GET /licenses/{key}/schedule (Phase 3, conditional) ───────────────
+
+// One Block of the target payload. `day` is "" for daily schedules and
+// "mon".."sun" for weekly schedules. Times are wall-clock minutes.
+struct TargetBlock {
+    std::string id;
+    std::string day; // "" for daily
+    int start_minutes = 0;
+    int end_minutes = 0;
+    bool overnight = false;
+    int duration_minutes = 0;
+    bool dynamic_audio = true;
+    std::vector<std::string> proclamation_ids;
+};
+
+// The target schedule payload (version/type/daily/weekly).
+struct SchedulePayload {
+    std::string version;
+    // "daily", "weekly", or "" for no schedule (type null).
+    std::string type;
+    std::vector<TargetBlock> daily;
+    // Weekly blocks keyed by day (mon..sun); empty for daily/none.
+    std::vector<TargetBlock> weekly;
+    // Legacy `blocks` flattening carried for backward compatibility.
+    std::vector<ScheduleBlock> legacy_blocks;
+
+    bool hasSchedule() const { return type == "daily" || type == "weekly"; }
+};
+
+// Parse the target payload (with version/type/daily/weekly). `blocks` (legacy)
+// is also read so older responses still populate `legacy_blocks`. Returns false
+// on malformed input.
+bool parseSchedulePayloadJson(const std::string& body, SchedulePayload& out);
+
+// Result of a conditional schedule fetch.
+enum class ScheduleFetchStatus {
+    Ok,            // 200; payload replaced
+    NotModified,   // 304; caller keeps its cached payload
+    NotFound,      // 404 unknown license
+    NetworkError,  // transport/other failure
+};
+
+// Fetch a License Key's schedule with conditional GET. When `etag` is
+// non-empty it is sent as If-None-Match. On success `out` is replaced and
+// `outEtag` receives the response ETag. Never throws.
+ScheduleFetchStatus fetchScheduleForLicense(const std::string& license_key,
+                                            const std::string& etag,
+                                            SchedulePayload& out,
+                                            std::string& outEtag);
+
 } // namespace radiantwave::api