feat(realtime): eager blocking pipeline warm-up + /backend/load API

[richiejp]

Description

Load models before a realtime session starts so the first request completes with the expected latency.

Also add an explicit model loading API and UI button, so clients with admin privileges can warmup models
before a session is even requested and without making probe requests where the intent of a request
may not be clear in the traces.

Notes for Reviewers

Realtime sessions previously lazy-loaded each pipeline sub-model (VAD,
transcription, LLM, TTS) on first use, so every cold session paid a
per-request model-load stall and load errors only surfaced mid-stream.

Warm the whole pipeline eagerly and blockingly at session start
(including the voice-gate speaker-recognition model, which an enforced
gate blocks each utterance on; compaction's summary_model stays lazy
since it only runs off the response path):

• Add backend.PreloadModel / PreloadModelByName as the single load path
  for every modality (no transcription special-case; backend-omitted
  configs are deprecated).
• The realtime session blocks on Model.Warmup and returns a
  model_load_error to the client if any stage fails to load;
  updateSession warms in the background. Opt out per pipeline with
  pipeline.disable_warmup, exposed as a UI toggle via the
  config-metadata registry.

Add a LocalAI-native POST /backend/load (and /v1/backend/load) that
pre-loads a model -- expanding realtime pipelines into their sub-models
-- as the inverse of /backend/shutdown. There is one preload engine
(backend.PreloadStages): the realtime Warmup methods, /backend/load and
the --load-to-memory startup flag all use it, so --load-to-memory now
also expands pipeline models and records load-failure traces. Pipeline
sub-model alias resolution is likewise shared
(ModelConfigLoader.LoadResolvedModelConfig). Surface the endpoint
everywhere an admin manages models:

• MCP admin tool load_model (httpapi + inproc clients, safety/catalog
  prompts, catalog/dispatch tests).
• "Load into memory" action in the React models UI.
• Swagger regenerated; docs moved to the general backend-monitor page
  since it is not realtime-specific.

Fix a Traces UI crash ("json: unsupported value: -Inf"): audio-snippet
RMS/peak now floor at a finite dBFS, and backend-trace data is sanitized
to drop non-finite floats before marshaling. The sanitizer is
copy-on-write -- it runs on every RecordBackendTrace, so containers are
only re-allocated on the paths that actually changed.

Migrate core/http/openresponses_test.go onto the prebuilt mock-backend
the rest of the http suite already uses -- it was the last spec still
pointing at a real HuggingFace model, so it 404'd wherever no vision
backend was built -- and fix its item_reference specs to send the
spec's "id" field instead of "item_id", which the handler never
accepted.

Assisted-by: Claude:claude-opus-4-8 Claude Code
Signed-off-by: Richard Palethorpe io@richiejp.com

Signed commits

• Yes, I signed my commits.

[mudler]

nice!