docs: Add comprehensive component architecture documentation

Created detailed architectural documentation showing component interactions,
communication patterns, and data flows across all StellaOps services.

## New Documentation

**docs/ARCHITECTURE_DETAILED.md** - Comprehensive architecture guide:
- Component topology diagram (all 36+ services)
- Infrastructure layer details (PostgreSQL, Valkey, RustFS, NATS)
- Service-by-service catalog with responsibilities
- Communication patterns with WHY (business purpose)
- 5 detailed data flow diagrams:
  1. Scan Request Flow (CLI → Scanner → Worker → Policy → Signer → Attestor → Notify)
  2. Advisory Update Flow (Concelier → Scheduler → Scanner re-evaluation)
  3. VEX Update Flow (Excititor → IssuerDirectory → Scheduler → Policy)
  4. Notification Delivery Flow (Scanner → Valkey → Notify → Slack/Teams/Email)
  5. Policy Evaluation Flow (Scanner → Policy.Gateway → OPA → PostgreSQL replication)
- Database schema isolation details per service
- Security boundaries and authentication flows

## Updated Documentation

**docs/DEVELOPER_ONBOARDING.md**:
- Added link to detailed architecture
- Simplified overview with component categories
- Quick reference topology tree

**docs/07_HIGH_LEVEL_ARCHITECTURE.md**:
- Updated infrastructure requirements section
- Clarified PostgreSQL as ONLY database
- Emphasized Valkey as REQUIRED (not optional)
- Marked NATS as optional (Valkey is default transport)

**docs/README.md**:
- Added link to detailed architecture in navigation

## Key Architectural Insights Documented

**Communication Patterns:**
- 11 communication steps in scan flow (Gateway → Scanner → Valkey → Worker → Concelier → Policy → Signer → Attestor → Valkey → Notify → Slack)
- PostgreSQL logical replication (advisory_raw_stream, vex_raw_stream → Policy Engine)
- Valkey Streams for async job queuing (XADD/XREADGROUP pattern)
- HTTP webhooks for delta events (Concelier/Excititor → Scheduler)

**Security Boundaries:**
- Authority issues OpToks with DPoP binding (RFC 9449)
- Signer enforces PoE validation + scanner digest verification
- All services validate JWT + DPoP on every request
- Tenant isolation via tenant_id in all PostgreSQL queries

**Database Patterns:**
- 8 dedicated PostgreSQL schemas (authority, scanner, vuln, vex, scheduler, notify, policy, orchestrator)
- Append-only advisory/VEX storage (AOC - Aggregation-Only Contract)
- BOM-Index for impact selection (CVE → PURL → image mapping)

This documentation provides complete visibility into who calls who, why they
communicate, what data flows through the system, and how security is enforced
at every layer.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/07_HIGH_LEVEL_ARCHITECTURE.md

@@ -50,15 +50,29 @@
 | **Web UI**                      | `stellaops/ui`               | Angular app for scans, diffs, policy, VEX, vulnerability triage (artifact-first), audit bundles, **Scheduler**, **Notify**, runtime, reports.          | Stateless.                                         |
 | **StellaOps.Cli**               | `stellaops/cli`              | CLI for init/scan/export/diff/policy/report/verify; Buildx helper; **schedule** and **notify** verbs.                                                 | Local/CI.                                          |
 
-### 1.2 Third‑party (self‑hosted)
+### 1.2 Infrastructure Requirements
 
-* **Fulcio** (Sigstore CA) — issues short‑lived signing certs (keyless).
-* **Rekor v2** (tile‑backed transparency log).
-* **RustFS** — offline-first object store with deterministic REST API; S3/MinIO compatibility layer available for legacy deployments.
-* **PostgreSQL** (≥16) — primary control-plane storage with per-module schema isolation (authority, vuln, vex, scheduler, notify, policy, concelier). See [Database Architecture](#database-architecture-postgresql).
-* **Valkey** (≥8.0) — Redis-compatible cache for DPoP nonces, event streams, queues, and rate limiting.
-* **Queue** — Valkey Streams (default); NATS JetStream available as optional transport (opt-in only).
-* **OCI Registry** — must support **Referrers API** (discover SBOMs/signatures).
+**REQUIRED Infrastructure:**
+
+* **PostgreSQL** (≥16) — **ONLY** database for all persistent data. Per-module schema isolation (authority, vuln, vex, scanner, scheduler, notify, policy, orchestrator). See [Database Architecture](#database-architecture-postgresql).
+* **Valkey** (≥8.0) — Redis-compatible cache, DPoP nonces (RFC 9449), event streams, job queues, rate limiting. **REQUIRED** for platform operation.
+* **RustFS** — S3-compatible object storage for SBOM artifacts, proof bundles, and scan evidence. HTTP API with deterministic responses.
+
+**OPTIONAL Infrastructure:**
+
+* **NATS JetStream** — Alternative messaging transport (Valkey Streams is default). Opt-in only via configuration.
+
+**External Dependencies:**
+
+* **OCI Registry** — Must support **Referrers API** (discover SBOMs/signatures).
+* **Fulcio** (Sigstore CA) — Issues short-lived signing certs (keyless signing). Optional if using KMS keys.
+* **Rekor v2** — Tile-backed transparency log. Optional if `OFFLINEKIT_ENABLED=true` (airgap mode).
+
+**Architecture Note:**
+- PostgreSQL is the ONLY database (MongoDB fully removed as of 2025-12-23)
+- Valkey replaces Redis (drop-in compatible, but required)
+- RustFS is primary object storage (MinIO removed)
+- NATS is OPTIONAL, not required (Valkey Streams handle queuing)
 
 ### 1.3 Cloud licensing (Stella Ops)