Document `Authorization: Bearer <token>` for existing Chatwoot API
tokens, available from v4.19.0. The OpenAPI auth option explains where
to obtain user, agent bot, and platform app tokens while retaining
`api_access_token` as a legacy option.
Follow-up to https://github.com/chatwoot/chatwoot/pull/15941. Related:
https://github.com/chatwoot/chatwoot/issues/1615 and
https://linear.app/chatwoot/issue/CW-2866/chore-standardise-the-access-token-based-authentication-in-chatwoot.
## Why
API consumers and generated clients should be able to discover the
standard Bearer header and reuse their existing tokens without changing
permissions.
## What this change does
- Adds HTTP Bearer as a separate authentication alternative wherever
existing API tokens are accepted.
- Explains that Bearer authentication is available from Chatwoot v4.19.0
and where to get user, agent bot, and platform app tokens.
- Recommends Bearer auth while keeping the `api_access_token` header
supported ahead of a future deprecation.
- Updates the attachment request example and generated OpenAPI JSON.
## Validation
Open the generated API documentation and confirm Bearer and legacy API
key options appear. On a v4.19.0 instance, authorize with an existing
API token and call an allowed endpoint. Confirm the attachment example
uses `Authorization: Bearer`.
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