feat(devex): product isolation and architecture safety

Context

The monorepo's product isolation model has matured — tach-enforced boundaries, facades, contract checks, product folder structure. But the scope is wider than just folder layout. DB-level isolation per product, automatic team scoping to prevent IDOR vulnerabilities, and the ongoing migration of legacy models all fall under the same umbrella: making products truly independent so teams can ship without breaking each other.

Goal

Products are isolated at every layer — code boundaries, test suites, databases, and queryset scoping — with enforcement that catches violations automatically.

Scope / ideas

• Continue migrating legacy models into product apps (dashboards, feature flags next in line)
• Harden multi-database routing per product (landed in #51896, denormalization in progress)
• Implement automatic team scoping to prevent cross-tenant data access (#47065)
• Maintain and extend tach boundary checks, contract checks, and product lint rules
• Product isolation skill and agent guidance for teams bootstrapping new products

Current state

Multi-database setup landed and re-landed (#51267 → #51896). Surveys, messaging, links, experiments models already migrated. Dashboards (#51924) and feature flags (#51923) in progress. Tach interfaces enforced on bootstrapped products (#50746). Contract checks run per-product in Turborepo (#50220). Team scoping issue (#47065) is open — currently relies on developer discipline and semgrep rules.

References

• https://github.com/PostHog/posthog/blob/master/products/README.md
• https://github.com/PostHog/posthog/pull/51896
• https://github.com/PostHog/posthog/issues/47065
• https://github.com/PostHog/posthog/pull/51924
• https://github.com/PostHog/posthog/pull/51923