feat: add getDocumentBytes endpoint for OCR + SignatureRequest context

- New POST /api/v1/SmartAssistant/action/getDocumentBytes returns
  {success, fileName, mimeType, sizeBytes, base64} for use by
  shira-hermes Claude Vision OCR fallback.
- DocumentAnalyzer: new getDocumentBytes() method + guessMimeType helper.
- CaseContextBuilder: expose signatureRequests in case context (for
  DigitalSignature integration — gracefully empty if entity absent).
- README: sync version badge.
- Version bump 2.7.3 → 2.8.0 (new public endpoint).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

manifest.json

@@ -3,11 +3,11 @@
     "module": "SmartAssistant",
     "description": "Unified AI Assistant for Legal CRM — floating chat with case memory, office alerts, and AI Gateway integration",
     "author": "klear",
-    "version": "2.7.3",
+    "version": "2.8.0",
     "acceptableVersions": [
         ">=8.0.0"
     ],
-    "releaseDate": "2026-04-14",
+    "releaseDate": "2026-04-19",
     "php": [
         ">=8.1"
     ]