Track forwarding notifications

[roznawsk]

Description

Describe your changes.

Motivation and Context

Why is this change required? What problem does it solve? If it fixes an open
issue, please link to the issue here.

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

[linear]

FCE-2859 Add notifications for Fishjam track forwardings

[Copilot]

Pull request overview

Adds new server notification types to represent “track forwarding” lifecycle events and updates the Elixir protobuf toolchain/docs accordingly.

Changes:

• Bump protobuf Elixir dependency from 0.15.x to 0.16.x and refresh lockfile.
• Extend ServerMessage with TrackForwarding / TrackForwardingRemoved messages (proto + generated .pb.ex).
• Regenerate/update markdown docs and README instructions to reflect the new protobuf tooling/version.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
fishjam_protos/mix.lock	Locks protobuf to 0.16.0.
fishjam_protos/mix.exs	Bumps protobuf dependency requirement to ~> 0.16.0.
fishjam_protos/lib/fishjam/server_notifications.pb.ex	Regenerated Elixir modules for new notifications + updated field metadata/deprecations.
fishjam/server_notifications.proto	Adds new TrackForwarding* messages and reorganizes oneof (also introduces new deprecations).
doc/docs.md	Adds documentation sections for the new messages and updates the ServerMessage oneof table.
README.md	Updates protobuf plugin install command to 0.16.0.

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