Add ElevenLabs as a Provider for Text to Speech

[jeffpaul]

Is your enhancement related to a problem? Please describe.

Following up on #986 and PR #991, we should add ElevenLabs as a supported Text-to-Speech (TTS) provider. This enhancement leverages the new provider architecture introduced in #991 (hopefully making the integration effort here a bit easier ;).

Noting @dkotter's comment in that PR:

Note: probably worth a followup to add ElevenLabs as a Text to Speech Provider once this is merged in, as that will be pretty easy at that point.

• Add ElevenLabs as an available Text-to-Speech provider in settings with respective ElevenLabs API credentials.
• Support selecting from available ElevenLabs voices within the provider settings UI.
• Integrate with the existing TTS generation flows, ensuring parity with other providers in terms of:
  • Generating audio from a post
  • Downloading the generated file
  • Displaying the audio file on posts/pages via the frontend "read to me" feature
• Include helpful inline documentation and a link to ElevenLabs’ API docs.

Designs

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Describe alternatives you've considered

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