fix: harden webhook reliability, fix bugs, add test suite

- Statuspage webhook always returns 200 to prevent subscriber removal
- Fix parseKvKey returning string chatId instead of number
- Queue consumer retries on Telegram 5xx instead of acking (prevents message loss)
- Fix observability top-level enabled flag (false → true)
- Add defensive null checks for webhook payload body
- Cache Bot instance per isolate to avoid middleware rebuild per request
- Add vitest + @cloudflare/vitest-pool-workers with 31 tests
- Document DLQ and KV sharding as declined features

wrangler.jsonc

@@ -25,7 +25,7 @@
 		]
 	},
 	"observability": {
-		"enabled": false,
+		"enabled": true,
 		"head_sampling_rate": 1,
 		"logs": {
 			"enabled": true,