feat: store session and exports in redis for cross-device portability

Replace the device-bound Telethon .session file and local CSV folders with
Redis-backed storage so the tool runs on any device from just .env.

- StringSession persisted at <prefix>:session:<phone> (no re-login per device)
- each group export saved as one self-contained JSON key per run:
  <prefix>:run:<phone>:<yyyymmddhhmmss>:<group_id>
- REDIS_PREFIX namespaces all keys; applied via a single key() helper
- no cross-key references: deleting any key cannot corrupt another's state
- list_exports() lists history, tolerant of deleted/corrupt keys

.env.example

@@ -2,3 +2,5 @@ PHONE=+1234567890
 API_ID=123456789
 API_HASH=0123456789abcdef0123456789abcdef
 GROUP_IDS=1234567890,9876543210
+REDIS_URL=rediss://default:<password>@<host>:<port>
+REDIS_PREFIX=telegram-export