refactor: split src into domain / application / infrastructure layers

Replace the history-driven lib/ + utils/ split with DDD layers:
- domain/: aggregate, repositories, value objects, pure parsers/format
- application/: feed-service, email-processor, feed-fetcher, stats
- infrastructure/: logging, auth, KV/R2 adapters, HTTP, framework glue

Pure file relocation; imports updated mechanically. Behaviour unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/infrastructure/logger.ts

@@ -0,0 +1,26 @@
+type LogLevel = "info" | "warn" | "error" | "debug";
+
+function log(
+  level: LogLevel,
+  message: string,
+  data?: Record<string, unknown>,
+): void {
+  const entry = data ? { level, message, ...data } : { level, message };
+  const line = JSON.stringify(entry);
+  if (level === "error" || level === "warn") {
+    console.error(line);
+  } else {
+    console.log(line);
+  }
+}
+
+export const logger = {
+  info: (message: string, data?: Record<string, unknown>) =>
+    log("info", message, data),
+  warn: (message: string, data?: Record<string, unknown>) =>
+    log("warn", message, data),
+  error: (message: string, data?: Record<string, unknown>) =>
+    log("error", message, data),
+  debug: (message: string, data?: Record<string, unknown>) =>
+    log("debug", message, data),
+};