feat: Document completed tasks across multiple components

- Added completed tasks documentation for Scheduler WebService, ImpactIndex, Models, Queue, Storage.Mongo, Worker, Signals, Signer, UI, Zastava.Observer, Zastava.Webhook, Zastava.Core, Cryptography.Kms, Cryptography, and Plugin.
- Each task includes ID, status, owners, dependencies, descriptions, and exit criteria to ensure clarity and traceability.
- Enhanced integration and unit testing coverage across various components to validate functionality and compliance with specifications.

docs/04_FEATURE_MATRIX.md

@@ -1,5 +1,7 @@
-# 4 · Feature Matrix — **Stella Ops**  
-*(rev 2.0 · 14 Jul 2025)*
+# 4 · Feature Matrix — **Stella Ops**  
+*(rev 2.0 · 14 Jul 2025)*
+
+> **Looking for a quick read?** Check [`key-features.md`](key-features.md) for the short capability cards; this matrix keeps full tier-by-tier detail.
 
 | Category               | Capability                            | Free Tier (≤ 333 scans / day) | Community Plug‑in | Commercial Add‑On   | Notes / ETA                                |
 | ---------------------- | ------------------------------------- | ----------------------------- | ----------------- | ------------------- | ------------------------------------------ |