Add technical explanations to advanced demo and convert all diagrams to Mermaid

- Add how/why technical breakdowns to every step in demo-advanced.md:
  handler→service→repository code paths, SQL details, security reasoning,
  field-by-field explanations, and architectural design decisions
- Convert all ASCII box diagrams to Mermaid across docs:
  architecture.md (9 diagrams), demo-advanced.md (6), concepts.md (1)
- Diagram types: flowcharts, sequence diagrams, ER diagram, state machine
- Remove placeholder Support & Community section from README
- Zero ASCII box-drawing characters remaining in docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/concepts.md

@@ -80,10 +80,18 @@ Targets are the systems where certificates actually get installed — NGINX web
 
 Every managed certificate in certctl goes through these states:
 
-```
-Pending → Active → Expiring → (auto-renewal) → Active → ...
-                              → Expired (if renewal fails)
-                              → Failed (if issuance fails)
+```mermaid
+stateDiagram-v2
+    [*] --> Pending: Certificate created
+    Pending --> Active: Issuance succeeds
+    Pending --> Failed: Issuance fails
+    Active --> Expiring: Within renewal window
+    Expiring --> RenewalInProgress: Auto-renewal triggered
+    RenewalInProgress --> Active: Renewal succeeds
+    RenewalInProgress --> Failed: Renewal fails
+    Expiring --> Expired: Renewal not attempted / all retries exhausted
+    Active --> Archived: Decommissioned
+    Failed --> Pending: Retry requested
 ```
 
 - **Pending**: Certificate record created, awaiting initial issuance