Add MongoDB storage library and update acceptance tests with deterministic stubs

- Created StellaOps.Notify.Storage.Mongo project with initial configuration.
- Added expected output files for acceptance tests (at1.txt to at10.txt).
- Added fixture input files for acceptance tests (at1 to at10).
- Created input and signature files for test cases fc1 to fc5.

docs/process/evidence-suppression.schema.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "Evidence Suppression (Stub)",
+  "type": "object",
+  "properties": {
+    "evidence_id": {"type": "string"},
+    "suppression_reason": {"type": "string"},
+    "justification": {"type": "string"},
+    "expiry": {"type": "string", "format": "date-time"},
+    "visibility": {"type": "string", "enum": ["private", "tenant", "public"]}
+  },
+  "required": ["evidence_id", "suppression_reason", "visibility"]
+}