v0.1.0

First release, targeting the Unicom360 API contract v0.1.0.

Added

• Unicom360.client/1 and an immutable %Unicom360.Client{}, with options from the keyword
  list, the application environment and the environment variables.
• Every operation of the public API, each returning {:ok, result} or
  {:error, %Unicom360.Error{}} with a ! variant: Unicom360.SMS.send/3,
  Unicom360.Email.send/3, Unicom360.WhatsApp.send_template/3,
  Unicom360.WhatsApp.send_session/3, Unicom360.WhatsApp.status/3,
  Unicom360.WhatsApp.templates/2, Unicom360.Balance.get/2, Unicom360.Balance.all/2 and
  Unicom360.health/2.
• Typed result structs that keep unknown fields (extra) and the parsed JSON (raw), with lookup
  helpers (by_number, by_address, by_id, by_name, accepted, invalid, rejected).
  Balances are Decimals.
• Builders for template parameters (TemplateHeader, TemplateBody, TemplateButton,
  TemplateRecipient) and session messages (Unicom360.WhatsApp.Session).
• One error struct, %Unicom360.Error{}, with Unicom360.Error.class/1 naming the canonical error
  classes, and codes normalised from the gateway's messages (Unicom360.Codes).
• Safe automatic retries (429, failures before sending, and 5xx, failures after sending and
  SEND_FAILED on retry-safe calls only), with UTC-minute waits after a 429 and full-jitter
  backoff otherwise; the auto_reference_ids option.
• Webhook verification and parsing (Unicom360.Webhook.verify/4, construct_event/4), the unsigned
  SMS and email delivery reports (parse_sms_callback/1, parse_email_callback/1), and the
  optional Plug adapter (Unicom360.Webhook.Plug, Unicom360.Webhook.BodyReader). A secret that is
  nil, empty or only whitespace is a :configuration_error before any other check, because anyone
  can forge a signature with an empty key; the plug raises in init/1 for a string, and answers 500
  without calling the handler for an {m, f, a}.
• Opt-in GSM-7 helpers (Unicom360.GSM).
• Requests through Req with its retries and redirects off, a transport seam through :plug and
  :req_options, and Unicom360.TransportError for custom adapters.
• Logging through Logger, with the API key and webhook secrets redacted everywhere.
• The contract's conformance suite (v0.1.0), a Prism contract test, examples for every operation
  and webhook receivers for Phoenix and plain Plug.