feat(litigationplanner): Berufung tile UX — collapse side selectors + appeal-target trigger label (t-paliad-301, m/paliad#132)

Two bugs from the Slice B1 Berufung rollout, one fix surface:

Bug A — duplicate side selectors collapse into ONE proactive-side
picker with per-proceeding role labels. The Verfahrensablauf used to
show both ?side= (Klägerseite/Beklagtenseite) AND ?appellant= (same
labels in case-form) on the Berufung tile. Now: one side picker, with
labels that swap to Berufungskläger/Berufungsbeklagter on the unified
upc.apl.unified tile (and Antragsteller/Antragsgegner Nichtigkeit on
upc.rev.cfi, Einsprechende(r)/Patentinhaber(in) on epa.opp.*).

Bug B — 'Auslösendes Ereignis' label derives from appeal_target on
the unified Berufung tile (5 target-specific strings) instead of the
proceeding's own trigger_event_label. Endentscheidung (R.118) /
Kostenentscheidung / Anordnung / Entscheidung im
Schadensbemessungsverfahren / Anordnung der Bucheinsicht.

Migration 137 (additive, no triggers on proceeding_types — verified
via mcp__supabase__execute_sql before drafting; no updated_at on the
table — lesson from mig 134 HOTFIX 3; no audit_reason setup needed):
  - ADD COLUMN role_proactive_label_de  (text NULL)
  - ADD COLUMN role_proactive_label_en  (text NULL)
  - ADD COLUMN role_reactive_label_de   (text NULL)
  - ADD COLUMN role_reactive_label_en   (text NULL)
  - Audit-first DO block lists the rows the UPDATE will touch.
  - Backfill 4 proceedings (upc.apl.unified + upc.rev.cfi +
    epa.opp.opd + epa.opp.boa); every other proceeding stays NULL
    and the renderer falls back to default labels.
  - Down drops the 4 columns.

Package additions (pkg/litigationplanner):
  - ProceedingType gains 4 *string fields (RoleProactive/Reactive
    LabelDE/EN) — db tags match the new columns; existing scans pick
    them up via the proceedingTypeColumns extension.
  - TriggerEventLabelForAppealTarget(target, lang) — Go-side map of
    the 5 appeal-target slugs to their DE/EN trigger-event labels.
    Empty result on unknown target signals "fall back to proceeding's
    own trigger_event_label".
  - Engine override: when CalcOptions.AppealTarget is set, the
    resulting Timeline.TriggerEventLabel/EN are replaced from the
    per-target map.

Frontend:
  - Removed #appellant-row div (was a separate 3-radio selector
    duplicating side).
  - Dropped ?appellant= URL state + the change handler + the init
    readback. The engine still consumes "appellant" — sourced from
    currentSide for role-swap proceedings; null otherwise.
  - applyRoleLabels(proceedingType) swaps the side-row radio labels
    from a hardcoded ROLE_LABELS map mirroring mig 137's backfill.
    Falls back to deadlines.side.claimant/defendant i18n keys for
    proceedings without overrides.
  - syncTriggerEventLabel reads data.triggerEventLabel from the calc
    response — which the engine override now sets per appeal_target,
    so no client-side mapping needed.
  - i18n cleanup: removed orphan deadlines.appellant.* keys (label /
    claimant / defendant / none) in both DE + EN.

Tests:
  - pkg/litigationplanner/appeal_target_label_test.go pins the 5×2
    label matrix + a coverage test that fails if a new entry in
    AppealTargets is added without populating the label switch.

Acceptance:
  - go build + go test all green (incl. new lp test).
  - bun run build clean (i18n codegen drops 4 keys, regenerates).
  - Live-DB audit before drafting confirmed: 4 target columns don't
    exist on proceeding_types, zero triggers on the table, exact
    column inventory matches the design.

pkg/litigationplanner/engine.go

@@ -571,6 +571,21 @@ func Calculate(
 	if pickedProceeding.TriggerEventLabelEN != nil {
 		resp.TriggerEventLabelEN = *pickedProceeding.TriggerEventLabelEN
 	}
+	// t-paliad-301 / m/paliad#132 Bug B — appeal_target-driven trigger
+	// label. When the request narrows to a specific appeal target, the
+	// "Auslösendes Ereignis" label describes the underlying decision
+	// (Endentscheidung / Kostenentscheidung / Anordnung /
+	// Schadensbemessung / Bucheinsicht) rather than the appeal
+	// proceeding itself. Overrides the proceeding's own
+	// trigger_event_label set above.
+	if opts.AppealTarget != "" {
+		if de := TriggerEventLabelForAppealTarget(opts.AppealTarget, "de"); de != "" {
+			resp.TriggerEventLabel = de
+		}
+		if en := TriggerEventLabelForAppealTarget(opts.AppealTarget, "en"); en != "" {
+			resp.TriggerEventLabelEN = en
+		}
+	}
 	if hasSubTrackNote {
 		resp.ContextualNote = subTrackNote.NoteDE
 		resp.ContextualNoteEN = subTrackNote.NoteEN