Add session-aware mic allocation re-run from casting diff

Summary

When an understudy or cover is playing a role for a given session, the existing mic auto-assignment should be re-run with the actual performers for that night. This adds a cast diff view to the Mics page, per-row apply/skip controls, a preview timeline, and a re-run modal with "preserve overrides" vs "wipe and regenerate" options.

Part 3 of 3 for issue #792. Depends on #1113 (casting matrix must exist).

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Backend changes

Update MicrophoneAutoAssignmentController (controllers/api/show/microphones.py, POST /show/microphones/suggest):

• Accept optional casting_overrides: {character_id: cast_id} in body — the algorithm uses these instead of Character.played_by for the affected characters' swap-cost calculations (same actor = zero swap cost)
• Accept preserve_overrides: bool (default true) — when false, existing manual allocations are not treated as locked slots (wipe mode)

Update utils/show/mic_assignment.py (minimal change):

• Add optional cast_overrides: dict[int, int] parameter to the assignment function
• Where cast identity is looked up via character.played_by, check the overrides dict first

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Frontend changes (client-v3/)

ConfigMics.vue — new re-run section:

• When castDiffForSession getter (from #1113) is non-empty: show amber "X cast changes pending — mic re-run recommended" badge in the page header
• Add CastDiffTable.vue section below existing tabs that auto-expands when there are pending changes

New CastDiffTable.vue (components/show/config/mics/):

• Header: "Pending Changes: {N}/{total}" with Select All / Clear
• Columns: checkbox | Mic# | Act · Scene | Character | Was (struck-through) | Now (bold) | Apply/Skip toggle
• Per-slot diff derived from current MicrophoneAllocation vs session casting
• Live preview via existing MicTimeline component (updates as checkboxes change)
• Footer: "{N} manual override(s) on file · Re-run with {checked} changes…" button (disabled if 0 checked)

New ReRunMicModal.vue (components/show/config/mics/):

• Two radio options:
  • Preserve manual overrides (default/recommended)
  • Wipe and regenerate from scratch (destructive)
• Lists current manual overrides
• On confirm: POST /api/v1/show/microphones/suggest with {casting_overrides, preserve_overrides} → PATCH /api/v1/show/microphones/allocations → success toast + badge clears

MicPlanStaleBanner.vue (full version, replacing stub from #1113):

• Shows exact slot count: "2 cast changes · 5 mic slots affected"

StartSessionConfirmModal.vue (from #1112/#1113) — full stale-mic blocking:

• If micPlanIsStaleForSession → amber warning: "Mic plan is out of date"; Start button disabled; "Go to Mics" link

stores/show.ts additions:

• Getter: micPlanIsStaleForSession(sessionId): boolean
• Action: reRunMicAllocation({sessionId, castingOverrides, preserveOverrides})

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E2E tests

Update client-v3/e2e/tests/09-show-config-mics.spec.ts:

• After cover casting change (from spec 12b), navigate to Mics
• "Pending Cast Changes" section visible with diff rows
• Uncheck one row → preview timeline updates
• "Re-run…" → select Preserve → confirm → toast + badge clears

Update client-v3/e2e/tests/13-live-show.spec.ts:

• Start session with cover cast → confirmation modal shows stale-mic warning + Start disabled
• Navigate to Mics, re-run → return → Start now enabled

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Verification

• pytest — mic assignment tests with casting_overrides; existing allocation tests unchanged
• npm run typecheck + npm run lint
• npm run test:e2e — full suite

Start with issue #1113 and the existing microphone assignment flow in controllers/api/show/microphones.py and utils/show/mic_assignment.py. Then inspect client-v3/ components, stores/show.ts, and the two named E2E specs to understand the casting diff, modal, stale-plan flow, and current tests. Done means the backend accepts the overrides, the Mics UI can preview and re-run selected changes, stale sessions are blocked, and the listed verification commands pass.