refactor: update API key source in .env.example and enhance README with badges and feature descriptions

feat: improve logging levels for HTTP clients and add detailed logging in premium and stars purchase methods
chore: remove unused transaction utility and adjust wallet processing logic
test: add end-to-end integration test for buying Stars

app/methods/ton.py

@@ -81,14 +81,23 @@ async def topup_ton(username: str, amount: int) -> dict:
         return {"success": False, "error": "Amount must be an integer >= 1 TON."}
 
     try:
+        logger.info("Loading session cookies")
         cookies = load_cookies()
+
+        logger.info("Fetching Fragment session hash")
         fragment_hash = await get_fragment_hash(cookies, HEADERS, ADS_PAGE)
+
+        # logger.info("Retrieving TON wallet info")
         account = await get_account_info()
 
         async with httpx.AsyncClient() as client:
+            logger.info("Searching recipient: %s", username)
             recipient = await search_ads_recipient(client, fragment_hash, cookies, username)
+
+            logger.info("Initializing topup request: %s TON to %s", amount, username)
             req_id = await init_ads_topup(client, fragment_hash, cookies, recipient, amount)
 
+            # logger.info("Requesting transaction payload (req_id=%s)", req_id)
             tx_data = {
                 "account": json.dumps(account),
                 "device": DEVICE,
@@ -101,6 +110,7 @@ async def topup_ton(username: str, amount: int) -> dict:
                 client, HEADERS, cookies, account, tx_data, fragment_hash
             )
 
+        logger.info("Broadcasting transaction to TON blockchain")
         tx_hash = await process_transaction(transaction)
         return {
             "success": True,