Build Stage 8 API v2 platform foundation

[anth-volk]

Fixes #3792

Summary

• Establish a dedicated, qualified Supabase/Postgres and private Storage foundation for the dormant API v2-alpha data domain.
• Add the reviewed canonical SQLModel schema, including stable reports with one-to-many versioned report runs and native UUID rerun idempotency.
• Add a mechanically separate, Postgres-only Alembic v2 chain with autogenerated schema and application-data migrations while preserving the v1 MySQL history.
• Replace writable runtime SQLite and per-container Redis with typed, expiring, shared managed-Redis cache repositories.
• Add deployment, startup, cache-loss, cross-instance, migration-lifecycle, and repository guard coverage.

Migration boundaries

• Covered routes and workflows: existing runtime-cache consumers, including computed households, tracers, AI analysis, reform impacts, budget-window coordination, and the historical /simulations response projection; v2 administrative migration and Storage bootstrap workflows; Cloud Run and App Engine cache wiring.
• Remaining on Flask/API v1: all production route ownership, Cloud SQL durable reads and writes, the existing Simulation Entrypoint, and current compute routing.
• Newly prepared: dormant Supabase Postgres/Storage, canonical SQLModel persistence, an isolated Alembic v2 lifecycle, managed cross-instance Redis, and associated CI and operational qualification.
• Intentional user-visible API changes: none. Existing response contracts and production data authority remain unchanged.

Operational behavior

• Redis contains only recoverable cache and bounded coordination state; durable domain data remains database-owned.
• Deployed modes fail closed without valid managed-cache configuration and never fall back to localhost or launch an embedded Redis server.
• Application imports, debug startup, and requests no longer create or initialize writable SQLite state.
• Cache loss is treated as a miss where recomputation is safe; coordination operations fail closed.
• The v2 schema remains dormant and is not activated for production request reads, writes, routes, or compute in Stage 8.

Verification

• OpenSpec implementation and strict verification: 86/86 tasks complete.
• Focused unit coverage: 205 tests passed.
• Disposable Postgres migration lifecycle: 3 tests passed, including downgrade and re-upgrade.
• Ruff lint and formatting checks passed for all changed Python files.
• scripts/run_quality_guards.py passed.
• scripts/export_migration_contracts.py completed with no generated drift.
• Changelog-fragment validation passed.

[codecov]

Codecov Report

❌ Patch coverage is 88.43813% with 228 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.20%. Comparing base (354903f) to head (5965b74).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/runtime_cache/repositories.py	81.65%	22 Missing and 18 partials ⚠️
policyengine_api/runtime_cache/fake.py	84.09%	13 Missing and 8 partials ⚠️
policyengine_api/runtime_cache/settings.py	84.49%	14 Missing and 6 partials ⚠️
policyengine_api/data/v2/migration_target.py	81.00%	10 Missing and 9 partials ⚠️
policyengine_api/data/v2/report_runs.py	82.02%	9 Missing and 7 partials ⚠️
policyengine_api/runtime_cache/client.py	51.72%	11 Missing and 3 partials ⚠️
policyengine_api/data/v2/storage_bootstrap.py	85.86%	6 Missing and 7 partials ⚠️
policyengine_api/runtime_cache/core.py	85.71%	4 Missing and 9 partials ⚠️
policyengine_api/data/v2/settings.py	90.00%	6 Missing and 4 partials ⚠️
policyengine_api/app_engine_runtime.py	81.81%	6 Missing and 2 partials ⚠️
... and 18 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3793      +/-   ##
==========================================
+ Coverage   84.19%   85.20%   +1.01%     
==========================================
  Files          86      108      +22     
  Lines        4720     6409    +1689     
  Branches      926     1112     +186     
==========================================
+ Hits         3974     5461    +1487     
- Misses        510      635     +125     
- Partials      236      313      +77     

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