Feat: Add production-grade multi-stage container image for API

Two-stage build (uv builder + python:3.12-slim runtime) with non-root
user (UID 1001), no dev deps, layer-cache-optimised dep install, and
graceful SIGTERM shutdown. Verified by api/tests/build/verify_production_image.sh
covering build, health endpoint, non-root, stdout logging, secret-free
layers, missing-env-var exit, and dep-layer cache hit. All 102 integration
tests still pass; shellcheck clean.

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

specs/010-api-prod-dockerfile/checklists/requirements.md

@@ -0,0 +1,34 @@
+# Specification Quality Checklist: Production-Grade API Container Image
+
+**Purpose**: Validate specification completeness and quality before proceeding to planning
+**Created**: 2026-05-07
+**Feature**: [spec.md](../spec.md)
+
+## Content Quality
+
+- [X] No implementation details (languages, frameworks, APIs)
+- [X] Focused on user value and business needs
+- [X] Written for non-technical stakeholders
+- [X] All mandatory sections completed
+
+## Requirement Completeness
+
+- [X] No [NEEDS CLARIFICATION] markers remain
+- [X] Requirements are testable and unambiguous
+- [X] Success criteria are measurable
+- [X] Success criteria are technology-agnostic (no implementation details)
+- [X] All acceptance scenarios are defined
+- [X] Edge cases are identified
+- [X] Scope is clearly bounded
+- [X] Dependencies and assumptions identified
+
+## Feature Readiness
+
+- [X] All functional requirements have clear acceptance criteria
+- [X] User scenarios cover primary flows
+- [X] Feature meets measurable outcomes defined in Success Criteria
+- [X] No implementation details leak into specification
+
+## Notes
+
+- All items pass. Ready for `/speckit-plan`.