docs: DevOps documentation — complete docs/devops/ set

Adds the full devops-documentation OpenSpec change implementation.
Separate from docs/developers/ — serves a different audience (operators,
not API consumers).

docs/devops/:
- README.md          — index and system overview
- architecture.md    — components, ports, data flow, Redis key patterns
- environment-variables.md — all 7 env vars (required + optional, formats, .env example)
- database.md        — 4-table schema, indexes, constraints, migration runner
- local-development.md — docker-compose setup, health checks, startup, Dockerfile gap noted
- security.md        — RSA key generation/rotation, CORS, bcrypt, secret storage guidance
- operations.md      — startup order, graceful shutdown, log reference, troubleshooting

QA gates: 48/48 tasks complete. All env vars verified against source.
All table names verified against migrations. All ports verified against
docker-compose.yml. All internal links resolve.

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

openspec/changes/devops-documentation/specs/system-overview/spec.md

@@ -0,0 +1,10 @@
+## ADDED Requirements
+
+### Requirement: System overview exists at docs/devops/README.md
+The system SHALL provide a `docs/devops/README.md` that serves as the entry point for DevOps engineers, including an index of all DevOps docs and a brief system overview.
+
+### Requirement: Architecture doc exists at docs/devops/architecture.md
+The system SHALL provide `docs/devops/architecture.md` documenting all components (Express server, PostgreSQL, Redis), their roles, ports, and data flow.
+
+### Requirement: Environment variable reference exists at docs/devops/environment-variables.md
+The system SHALL provide `docs/devops/environment-variables.md` documenting every environment variable with name, type, required/optional, default, and example value.