docs: remediate documentation gaps across quickstart, backup, config, API spec, and operations guides (#330)

- Fix Cyrillic character in quickstart image ref and correct registry to Docker Hub (saelix/sencho)
- Correct backup guide WAL references (Sencho uses SQLite default journal mode)
- Add SSL/TLS reverse proxy examples for Nginx, Traefik, and new Caddy configuration
- Add missing env vars (PORT, DATA_DIR, NODE_ENV, FRONTEND_URL, SSO_LDAP_DISPLAY_NAME) to .env.example
- Add upgrade & migration guide documenting automatic schema migrations
- Add self-hosting best practices (1:1 path rule, Docker socket security, resource recs)
- Add architecture overview (system design, request flow, database schema, multi-node model)
- Add development & contributor guide (setup, tests, code style, PR workflow)
- Update OpenAPI spec from v0.23.0 to v0.25.3 with Registries and Image Updates endpoints
- Update docs.json navigation with all new pages and API groups

docs/docs.json

@@ -121,14 +121,18 @@
             "group": "Reference",
             "pages": [
               "reference/settings",
-              "reference/security-advisories"
+              "reference/security-advisories",
+              "reference/architecture",
+              "reference/development"
             ]
           },
           {
             "group": "Operations",
             "pages": [
               "operations/troubleshooting",
-              "operations/backup"
+              "operations/backup",
+              "operations/upgrade",
+              "operations/self-hosting"
             ]
           }
         ]
@@ -233,6 +237,24 @@
               "GET /api/scheduled-tasks/{id}/runs",
               "GET /api/scheduled-tasks/{id}/runs/export"
             ]
+          },
+          {
+            "group": "Registries",
+            "pages": [
+              "GET /api/registries",
+              "POST /api/registries",
+              "PUT /api/registries/{id}",
+              "DELETE /api/registries/{id}",
+              "POST /api/registries/{id}/test"
+            ]
+          },
+          {
+            "group": "Image Updates",
+            "pages": [
+              "GET /api/image-updates",
+              "POST /api/image-updates/refresh",
+              "GET /api/image-updates/status"
+            ]
           }
         ]
       }