[lpukatch]

Managed channels currently rerank streams during generation even when Podium owns the intended order. This adds durable per-channel ordering ownership and PUT /api/v1/channels/by-dispatcharr/{id}/external-order for core/plugin event and team channels, plus an explicit return-to-Teamarr control in the channel list.

The endpoint validates full membership edits, commits ownership and recovery intent before the Dispatcharr write, confirms the live result, then finalizes positions, the order hash and memberships. Failed or interrupted writes remain fenced until an idempotent retry succeeds. Generation and reorder-only passes share a cross-process lock; the final stream-write boundary preserves surviving live order and appends new members across lifecycle, drift, pin and team/plugin paths.

Rollout requires Teamarr first, a shared database/lock directory across workers, and a single Teamarr manager for each Dispatcharr channel. Keep PODIUM_TEAMARR_EXTERNAL_ORDERING=false until Teamarr is deployed and smoke-tested. Unknown additions and disallowed windows return 422. Unmanaged acknowledgements are side-effect free; unmanaged adoption/remapping is operationally excluded during the direct-write handoff. Disabling Podium does not release stored ownership.

Validation:

• Ruff and frontend production build pass.
• 72 contract tests pass; 14 focused recovery/migration/unassign checks and the historical-membership regression also pass.
• Full backend suite is running in four isolated processes; results will be added before this PR is marked ready.

Bead: teamarr-76mo. Target: dev. Leave unmerged.