Add Phase 1a scale-readiness plan and update README

[droxey]

Motivation

• Provide an execution-ready plan to harden Phase 1a for scale without expanding product scope by closing gaps around SLOs, async boundaries, data partitioning, and runbooks.
• Make the scale-readiness guidance discoverable alongside existing MVP planning artifacts.

Description

• Add docs/scale-readiness-plan-2026-04-27.md with concrete SLO/SLI targets, error-budget policy, async side-effect worker recommendations (publish/label/tracking queues), idempotency and DLQ rules, data-model hardening, and a two-track delivery plan (pre-launch and post-launch).
• Update README.md to reference the new scale-readiness document for discoverability.
• Kept changes limited to documentation and discovery surface only, with no runtime or schema modifications.

Testing

• Verified repository changes and file contents using git status --short which showed the README update and the new docs/scale-readiness-plan-2026-04-27.md as staged/committed, and noted an unrelated untracked node_modules/ directory.
• Confirmed latest commit appears in history using git log -1 --oneline which returned the new change.
• Ran the PR creation tool validation which produced a PR title and body confirming the added document and README link were processed successfully.

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Codex Task