feat: Add CVSS receipt management endpoints and related functionality

- Introduced new API endpoints for creating, retrieving, amending, and listing CVSS receipts.
- Updated IPolicyEngineClient interface to include methods for CVSS receipt operations.
- Implemented PolicyEngineClient to handle CVSS receipt requests.
- Enhanced Program.cs to map new CVSS receipt routes with appropriate authorization.
- Added necessary models and contracts for CVSS receipt requests and responses.
- Integrated Postgres document store for managing CVSS receipts and related data.
- Updated database schema with new migrations for source documents and payload storage.
- Refactored existing components to support new CVSS functionality.

src/Concelier/__Libraries/StellaOps.Concelier.Storage.Postgres/Migrations/004_documents.sql

@@ -0,0 +1,23 @@
+-- Concelier Postgres Migration 004: Source documents and payload storage (Mongo replacement)
+
+CREATE TABLE IF NOT EXISTS concelier.source_documents (
+    id UUID NOT NULL,
+    source_id UUID NOT NULL,
+    source_name TEXT NOT NULL,
+    uri TEXT NOT NULL,
+    sha256 TEXT NOT NULL,
+    status TEXT NOT NULL,
+    content_type TEXT,
+    headers_json JSONB,
+    metadata_json JSONB,
+    etag TEXT,
+    last_modified TIMESTAMPTZ,
+    payload BYTEA NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    expires_at TIMESTAMPTZ,
+    CONSTRAINT pk_source_documents PRIMARY KEY (source_name, uri)
+);
+
+CREATE INDEX IF NOT EXISTS idx_source_documents_source_id ON concelier.source_documents(source_id);
+CREATE INDEX IF NOT EXISTS idx_source_documents_status ON concelier.source_documents(status);