feat(dsms): version chain history + diff endpoint + Audit Timeline UI

DSMS Stufe 3 — making the parent_cid chain useful end-to-end.

Gateway (dsms-gateway):
- /api/v1/documents/{cid}/history alias added next to the legacy
  /documents/{cid}/history (history endpoint itself was already there,
  just under an inconsistent prefix).
- NEW /api/v1/documents/{cid_a}/diff/{cid_b}: fetches both packages from
  IPFS, computes a metadata diff (per-field old/new), and renders a
  unified text diff for utf-8 payloads. Binary payloads return only
  metadata diff with a "binary — compare via rendered export" note.
- 4 new pytest cases (mocking ipfs_cat): text diff, binary fallback,
  fetch error, history chain depth — all green.

Frontend (admin-compliance):
- CIDHistoryModal: lazy-loads /dsms/documents/:cid/history, renders the
  version chain as a vertical timeline, marks the AKTUELL entry, and
  per-step exposes a "Diff zu V<n>" button that loads + renders the diff
  inline (metadata table + unified text diff in a monospace panel).
- AuditTimelinePage: existing CID badge now sits next to a "Verlauf
  anzeigen" link that opens the modal. Handles both Python's plain-CID
  audit values and the Go techfile flow's JSON envelope {cid, filename,
  size} via extractCID() helper.

This makes "show me how this CE-Akte changed between V2 and V3"
self-service in the UI instead of a curl-against-IPFS workflow.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

admin-compliance/app/sdk/audit-timeline/page.tsx

@@ -1,6 +1,8 @@
 'use client'
 
+import { useState } from 'react'
 import { useAuditTimeline, type AuditEntry } from './_hooks/useAuditTimeline'
+import CIDHistoryModal from './_components/CIDHistoryModal'
 
 const ENTITY_LABELS: Record<string, string> = {
   evidence: 'Nachweis', control: 'Control', document: 'Dokument',
@@ -16,8 +18,24 @@ const ACTION_COLORS: Record<string, string> = {
 
 const FILTER_OPTIONS = ['all', 'evidence', 'dsms_archive', 'control', 'document', 'dsfa', 'vvt', 'tom']
 
+// new_value may be a plain CID (from Python evidence flow) or a JSON envelope
+// {"cid":"X","filename":"...","size":"..."} (from the Go IACE tech-file flow).
+function extractCID(value: string): string {
+  const trimmed = value.trim()
+  if (trimmed.startsWith('{')) {
+    try {
+      const parsed = JSON.parse(trimmed)
+      if (typeof parsed.cid === 'string') return parsed.cid
+    } catch {
+      // fall through
+    }
+  }
+  return trimmed
+}
+
 export default function AuditTimelinePage() {
   const { entries, loading, filter, setFilter } = useAuditTimeline()
+  const [historyCID, setHistoryCID] = useState<string | null>(null)
 
   return (
     <div className="max-w-4xl mx-auto space-y-6">
@@ -58,16 +76,18 @@ export default function AuditTimelinePage() {
 
           <div className="space-y-4">
             {entries.map((entry) => (
-              <TimelineEntry key={entry.id} entry={entry} />
+              <TimelineEntry key={entry.id} entry={entry} onShowHistory={setHistoryCID} />
             ))}
           </div>
         </div>
       )}
+
+      {historyCID && <CIDHistoryModal cid={historyCID} onClose={() => setHistoryCID(null)} />}
     </div>
   )
 }
 
-function TimelineEntry({ entry }: { entry: AuditEntry }) {
+function TimelineEntry({ entry, onShowHistory }: { entry: AuditEntry; onShowHistory: (cid: string) => void }) {
   const dotColor = ACTION_COLORS[entry.action] || 'bg-gray-400'
   const isCID = entry.field_changed === 'dsms_cid' || entry.action === 'archive'
   const date = new Date(entry.performed_at)
@@ -94,7 +114,7 @@ function TimelineEntry({ entry }: { entry: AuditEntry }) {
               <p className="text-xs text-gray-600 dark:text-gray-400 mt-1">{entry.change_summary}</p>
             )}
             {isCID && entry.new_value && (
-              <div className="mt-2 flex items-center gap-2">
+              <div className="mt-2 flex items-center gap-2 flex-wrap">
                 <svg className="w-3.5 h-3.5 text-emerald-600 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor">
                   <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9 12l2 2 4-4m5.618-4.016A11.955 11.955 0 0112 2.944a11.955 11.955 0 01-8.618 3.04A12.02 12.02 0 003 9c0 5.591 3.824 10.29 9 11.622 5.176-1.332 9-6.03 9-11.622 0-1.042-.133-2.052-.382-3.016z" />
                 </svg>
@@ -102,6 +122,16 @@ function TimelineEntry({ entry }: { entry: AuditEntry }) {
                   {entry.new_value.length > 20 ? entry.new_value.slice(0, 8) + '...' + entry.new_value.slice(-6) : entry.new_value}
                 </code>
                 <span className="text-[10px] text-emerald-500">DSMS/IPFS</span>
+                <button
+                  onClick={(e) => {
+                    e.stopPropagation()
+                    if (entry.new_value) onShowHistory(extractCID(entry.new_value))
+                  }}
+                  className="text-[10px] text-purple-600 hover:text-purple-800 dark:text-purple-400 underline-offset-2 hover:underline"
+                  title="DSMS-Versionsverlauf und Diff zur Vorversion anzeigen"
+                >
+                  Verlauf anzeigen
+                </button>
               </div>
             )}
           </div>