Add unit and integration tests for VexCandidateEmitter and SmartDiff repositories

- Implemented comprehensive unit tests for VexCandidateEmitter to validate candidate emission logic based on various scenarios including absent and present APIs, confidence thresholds, and rate limiting.
- Added integration tests for SmartDiff PostgreSQL repositories, covering snapshot storage and retrieval, candidate storage, and material risk change handling.
- Ensured tests validate correct behavior for storing, retrieving, and querying snapshots and candidates, including edge cases and expected outcomes.

docs/quickstart.md

@@ -11,7 +11,7 @@
 | Resources | 2 vCPU / 2 GiB RAM / 10 GiB SSD | Fits developer laptops |
 | TLS trust | Built-in self-signed or your own certs | Replace `/certs` before production |
 
-Keep Redis and MongoDB bundled unless you already operate managed instances.
+Keep Redis and PostgreSQL bundled unless you already operate managed instances.
 
 ## 1. Download the signed bundles (1 min)
 
@@ -42,14 +42,14 @@ Create `.env` with the essentials:
 STELLA_OPS_COMPANY_NAME="Acme Corp"
 STELLA_OPS_DEFAULT_ADMIN_USERNAME="admin"
 STELLA_OPS_DEFAULT_ADMIN_PASSWORD="change-me!"
-MONGO_INITDB_ROOT_USERNAME=stella_admin
-MONGO_INITDB_ROOT_PASSWORD=$(openssl rand -base64 18)
-MONGO_URL=mongodb
+POSTGRES_USER=stella_admin
+POSTGRES_PASSWORD=$(openssl rand -base64 18)
+POSTGRES_HOST=postgres
 REDIS_PASSWORD=$(openssl rand -base64 18)
 REDIS_URL=redis
 ```
 
-Use existing Redis/Mongo endpoints by setting `MONGO_URL` and `REDIS_URL`. Keep credentials scoped to Stella Ops; Redis counters enforce the transparent quota (`{{ quota_token }}` scans/day).
+Use existing Redis/PostgreSQL endpoints by setting `POSTGRES_HOST` and `REDIS_URL`. Keep credentials scoped to Stella Ops; Redis counters enforce the transparent quota (`{{ quota_token }}` scans/day).
 
 ## 3. Launch services (1 min)