feat(litigationplanner): Berufung unification — one upc.apl + 5 appeal_target chips (Slice B1, m/paliad#124 §18.1)

Collapses the 3 UPC appeal proceeding_types (upc.apl.merits 7 rules,
upc.apl.cost 2, upc.apl.order 7 = 16 total across 3 codes) into ONE
unified upc.apl proceeding type + a per-rule applies_to_target[]
discriminator. The verfahrensablauf picker now shows one "Berufung"
tile; after picking it, the user selects which decision the appeal is
directed AT via a 5-chip group (Endentscheidung / Kostenentscheidung /
Anordnung / Schadensbemessung / Bucheinsicht) and the engine filters
rules whose applies_to_target contains the picked slug.

m's 2026-05-26 decision: Schadensbemessung-as-appeal is a NEW first-
class target with its OWN rule set (no shared inheritance from
merits). The 5 enum values are all defined + addressable; for now
schadensbemessung and bucheinsicht return empty timelines until rules
are seeded in a follow-up slice (likely via /admin/rules or pairing
with t-paliad-193 orphan-concept-seed).

Migration 134 (additive only):
  - ADD proceeding_types.appeal_target text (CHECK on 5 slugs OR NULL)
  - ADD deadline_rules.applies_to_target text[] (CHECK each element
    in the 5 slugs)
  - INSERT the unified upc.apl row (inherits sort/color from
    upc.apl.merits)
  - Audit-first RAISE NOTICE pass listing every row about to be
    touched + a post-migration sanity check
  - Reassign rule rows: merits → applies_to_target={endentscheidung},
    cost → {kostenentscheidung}, order → {anordnung}
  - Archive (is_active=false, NOT DELETE) the 3 old proceeding_types
    so historical FKs stay intact
  - Down migration restores is_active=true on the 3 old types, points
    rules back by their applies_to_target stamp, drops the unified
    row, drops both columns. Safe.

Package additions (pkg/litigationplanner):
  - AppealTarget* constants + AppealTargets[] ordered list +
    IsValidAppealTarget(s) predicate (silent no-op on unknown slugs
    so a stale frontend chip doesn't break the render)
  - ProceedingType.AppealTarget *string field (top-level marker;
    NULL on non-appeal proceedings)
  - Rule.AppliesToTarget pq.StringArray field (per-row applies-to set)
  - CalcOptions.AppealTarget string (engine filter — when set,
    keeps only rules whose AppliesToTarget contains the slug)

Engine filter runs after ApplyRuleOverrides but before the rule walk
so the existing condition_expr / spawn / appellant-context machinery
operates on the filtered subset transparently.

paliad-side wiring:
  - deadline_rule_service.go: ruleColumns + proceedingTypeColumns
    extended to scan the new columns
  - handlers/fristenrechner.go: AppealTarget JSON field on the
    request payload, threaded into CalcOptions

Frontend (verfahrensablauf surface only):
  - Single "Berufung" tile replaces the 3 separate Berufung tiles
  - New 5-chip appeal-target row, shown only when upc.apl is picked
  - URL state ?target=<slug>; default endentscheidung when none set
  - APPELLANT_AXIS_PROCEEDINGS updated: upc.apl.* (3 entries) →
    upc.apl (1 entry)
  - i18n keys (DE + EN) for the new tile + the 5 chip labels +
    the "Worauf richtet sich die Berufung?" / "Appeal against:" prompt
  - calculateDeadlines threads appealTarget through to the API

Acceptance:
  - go build clean, go test all green (existing test suite — no new
    tests on the engine filter as a follow-up; the migration's
    sanity-check DO block guards the rule-reassignment count)
  - Live audit before drafting confirmed: 3 active UPC appeal
    proceeding_types, 16 rules total, primary_party already conforms
    to 4-value vocab on all proceeding-bound rules

internal/handlers/fristenrechner.go

@@ -69,6 +69,14 @@ func handleFristenrechnerAPI(w http.ResponseWriter, r *http.Request) {
 		// stay in the result list. Default false preserves the legacy
 		// suppression. HiddenCount on the response is independent.
 		IncludeHidden bool `json:"includeHidden,omitempty"`
+		// Slice B1 / m/paliad#124 §18.1: narrows the unified UPC
+		// Berufung (upc.apl) timeline to the rule subset whose
+		// applies_to_target contains the requested slug. Empty = no
+		// filter. Valid values: endentscheidung | kostenentscheidung
+		// | anordnung | schadensbemessung | bucheinsicht. Unknown
+		// slugs are silently dropped (no filter) so a stale frontend
+		// chip doesn't 400 the request.
+		AppealTarget string `json:"appealTarget,omitempty"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "Ungültige Anfrage"})
@@ -116,6 +124,7 @@ func handleFristenrechnerAPI(w http.ResponseWriter, r *http.Request) {
 		SkipRules:        addendum.SkipRules,
 		IncludeCCRFor:    addendum.IncludeCCRFor,
 		IncludeHidden:    req.IncludeHidden,
+		AppealTarget:     req.AppealTarget,
 	})
 	if err != nil {
 		if errors.Is(err, services.ErrUnknownProceedingType) {