feat(inference): add parakeet.cpp backend

[sozercan]

What this PR does / why we need it:

Adds LocalAI v4.8.2 parakeet.cpp speech-to-text support for CPU and NVIDIA CUDA deployments.

• Supports Linux CPU on amd64 and arm64, plus CUDA 12 on amd64.
• Adds secure GGUF downloading, model-aware caching, and LocalAI configuration generation for runner images.
• Installs FFmpeg so Parakeet can normalize noncanonical audio inputs.
• Adds CPU and CUDA runner definitions, release matrices, and pinned transcription CI coverage.

Which issue(s) this PR fixes:

None.

Special notes for your reviewer:

Validation completed:

• go test -race -count=1 ./...
• golangci-lint run -v ./... --timeout 5m --new-from-rev=HEAD: zero issues
• actionlint and YAML parsing passed; the existing custom gpu runner label was allowed
• Built and ran the Linux arm64 CPU runner with LocalAI v4.8.2
• A stereo 44.1 kHz fixture produced the exact expected transcript
• Runtime hygiene checks passed and the compressed arm64 image is about 235 MiB, below its 250 MiB budget

CUDA execution requires the existing self-hosted GPU workflow. ROCm support is intentionally out of scope.

[Copilot]

Pull request overview

Adds LocalAI v4.8.2 Parakeet speech-to-text support for CPU and CUDA runners.

Changes:

• Adds Parakeet backend selection, platform validation, and FFmpeg support.
• Implements secure GGUF downloading, caching, and LocalAI configuration.
• Adds runner definitions, release matrices, and CPU/GPU transcription tests.

Show a summary per file

File	Description
runners/parakeet-cpp-cuda.yaml	Defines the CUDA runner.
runners/parakeet-cpp-cpu.yaml	Defines the CPU runner.
pkg/utils/const.go	Adds the backend constant.
pkg/build/build.go	Validates Parakeet platforms and runtimes.
pkg/build/build_test.go	Tests validation rules.
pkg/aikit2llb/inference/runner.go	Implements runtime model handling.
pkg/aikit2llb/inference/runner_test.go	Tests downloading, validation, and caching.
pkg/aikit2llb/inference/parakeet.go	Installs FFmpeg.
pkg/aikit2llb/inference/parakeet_test.go	Tests dependency installation.
pkg/aikit2llb/inference/convert.go	Selects the runtime base.
pkg/aikit2llb/inference/convert_test.go	Tests base selection.
pkg/aikit2llb/inference/backend.go	Selects and installs backend artifacts.
pkg/aikit2llb/inference/backend_test.go	Tests artifact selection and metadata.
.github/workflows/test-docker-runner.yaml	Adds CPU transcription CI.
.github/workflows/test-docker-runner-gpu.yaml	Adds CUDA transcription CI.
.github/workflows/release-runners.yaml	Adds release targets and size budgets.

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• Files reviewed: 16/16 changed files
• Comments generated: 1
• Review effort level: Balanced