feat(api): add runtime auxiliary scale connections

[MaxRink]

Summary

Adds runtime-only auxiliary scale connections for issue #833. ScaleController remains the sole primary connection. A new registry owns auxiliary sessions by device ID, including independent snapshots, tare routing, pending/closing reservations, cancellation fencing, and cleanup after delayed connection completion.

Generic connect accepts optional connectionRole: omitted means primary and auxiliary explicitly selects an external auxiliary. Inventory reports local connectionRole. PUT /api/v1/scales/{id}/tare and ws/v1/scales/{id}/snapshot support both primary and auxiliary IDs; legacy singular routes remain primary-only. Bengle’s integrated scale remains the primary scale while explicit external auxiliary discovery and connect remain available. No dosing settings, purposes, endpoints, or persisted role assignment are introduced.

Linked Issue

Related #833.

This change depends on the shared opaque path component boundary from #858. The multi-device Skale consumer is #859; the first-stage single-device driver remains in #846.

Verification

• Combined Linux Flutter gate for the current publication stack: 4,315 passed, 1 skipped, 0 failed; analyzer clean (48.0s). Source was unchanged during the gate.
• Runtime passed Bengle’s integrated scale as primary and MockScale as auxiliary, including tare, addressed/raw WebSocket frames, disconnect handling, and unchanged preferences.

Impact

Adds public role-aware connection and per-ID API behavior, OpenAPI/WebSocket specification updates, device-management documentation, and runtime lifecycle state held in memory only. Existing primary ScaleController behavior and legacy primary routes remain compatible. No database migration is required.

Contributor Responsibility

AI-assisted development is allowed. The submitter remains responsible for the submitted work.

• I have reviewed and understand all changes in this PR and take responsibility for their correctness, security, behavior, licensing, and provenance, including any AI-assisted or AI-generated work.

[tadelv]

The implementation direction looks sound and matches the architecture agreed on #833: ScaleController remains the only brewing/primary scale owner, auxiliaries are generic runtime-only sessions, the same physical ID cannot occupy both roles, Bengle keeps its integrated primary while allowing explicit external auxiliaries, and the new per-ID routes follow the opaque-ID boundary from #858. I did not find a substantiated code-level blocker in those paths.

There are still repository hard gates that prevent approval:

1. The required Contributor Responsibility acknowledgement is still unchecked in the PR body. AGENTS.md makes the required PR-template fields a hard pre-PR gate, and the current Contribution policy job fails on exactly this. Because that job fails first, format/analyze/tests/build are currently skipped in GitHub CI.
2. This is an external contribution (author_association: CONTRIBUTOR), and #833 currently has neither ready-for-agent nor ready-for-human. The contribution-policy workflow requires at least one referenced open issue with one of those labels. Related #833 is a valid reference, but after the acknowledgement is fixed the gate will still reject #833 until a maintainer marks the issue accepted (or the PR references another accepted issue).
3. #858 has now merged to main, while this branch is still based on the pre-#858 main and carries the #858 stack in its PR diff. Please update the branch onto current main and rerun the checks so CI validates the actual auxiliary-scale change against the dependency that is now canonical.

Once those gates are cleared and the real CI run is green, I do not currently see another change I would require from the implementation.