Isolate products/workflows for selective CI and move remaining team code out of posthog/

Problem

Every workflows-team PR runs the full backend suite. products/workflows, products/cdp, and products/messaging are not isolated products: none declares backend:contract-check, so turbo-discover cannot skip the full Django suite for them. The merge queue uses the same selection, so this cost lands on every Trunk run.

Some team-owned code also still lives under posthog/ (posthog/cdp/, messaging leftovers), where any change always triggers the full suite.

Goal

Move everything movable into products/, and isolate workflows (then messaging) so internal-only changes skip the full suite. Follow the isolating-product-facade-contracts skill and products/architecture.md. Use products/error_tracking as the reference implementation.

Recon (2026-08-18)

• hogli product:isolate:scan workflows: core-coupling count 35 (single-PR territory per the skill).
• hogli product:isolate:scan messaging: core-coupling count 10.
• hogli product:maturity workflows: 51/100. Models are fully in-product. There is no real facade. Views live in backend/api/ with 42 direct .objects calls. api/hog_flow.py is ~4,600 lines and most of it is business logic, not HTTP.
• Wiring is nearly free: no Temporal code, no query runners, Celery tasks already in backend/tasks/, max_tools.py already at the designated path.
• Sequencing constraint: creating facade/contracts.py flips hogli product:lint to strict mode, which fails on the non-canonical dirs backend/{api,providers,services,utils}/. Structure must become canonical before or with the facade.

Track W: isolate products/workflows

• W1: fold non-canonical dirs into logic/. Move backend/providers/, backend/services/, and backend/utils/ under backend/logic/. Rewrite the ~10 external dotted references (posthog/models/integration.py, posthog/tasks/integrations.py, posthog/api/integration.py, customer_analytics, ai_observability) and the @patch strings in posthog/models/test/test_integration_model.py. Add the root tsconfig.json. Lenient lint tolerates this, so it can land first.
• W2: move backend/api/ to backend/presentation/. Run hogli product:isolate:move workflows (dry-run first). It moves viewsets to presentation/views/, relocates api tests to backend/tests/api/, pins Celery task names, and rewrites dotted paths repo-wide. Reroute the two posthog/urls.py bypasses (SES events webhook, PublicHogFlowTemplateViewSet) through backend/routes.py. Thick views get ignore_imports TODO entries in pyproject.toml. This PR conflicts with every open workflows PR, so land it in a quiet window and regenerate on conflict.
• W3: facade + contracts + core-caller sweep. Add backend/facade/contracts.py (frozen pydantic dataclasses) and backend/facade/api.py with explicit mappers, plus facade/tasks.py re-exporting the two SES beat tasks. Sweep the 35 core references from the scan: HogFlow and TeamWorkflowsConfig model access becomes capability functions returning contracts, provider and integration-usage callers move to facade functions, test fixtures switch to apps.get_model with TYPE_CHECKING imports. Reroute the dotted HogFlow.State reference in posthog/settings/web.py to a facade path. Replace the 6 cross-product imports of products.cdp...test_hog_function_templates.MOCK_NODE_TEMPLATES with a non-test fixture home. Add workflows to the shared [[interfaces]] regex in tach.toml. Note: workflows cannot use a MODEL_CROSSINGS allowance (that list only shrinks), so HogFlow never crosses as a class.
• W4: presentation wave. Thin HogFlowViewSet (~1,900 lines, 21 actions): extract the perform_update write semantics (draft routing, staleness fences, select_for_update, revision bump, reschedule), secret partitioning and rehydration, publish-impact and token minting, and template resolution into logic/ behind facade functions, with parity tests. Do the same, smaller, for HogFlowTemplateViewSet. Done means zero workflows ignore_imports entries remain.
• W5: seal and enable. Add backend:contract-check to package.json and a products/workflows/turbo.json with inputs narrowed to backend/facade/**, backend/presentation/**, backend/routes.py, backend/tasks/**, backend/max_tools.py. hogli product:lint blocks this until W4 empties the allowlist. This is where the CI skip turns on.

Track M: isolate products/messaging

• Same recipe at coupling 10: restructure non-canonical dirs, build a facade for the MessageCategory / preferences / customerio sync / remote-config consumers, reroute the two posthog/urls.py webhook imports, sweep. Workflows imports messaging internals from api/hog_flow.py (design operations, unlayer, MessageTemplate), so messaging's facade should land before or with W3. Likely 1-2 PRs.

Track P: move posthog/ leftovers into products/

• P1: move posthog/models/messaging.py, posthog/models/message_assets/, posthog/models/hog_invocation_results/, and posthog/api/hog_invocation_*.py into the owning products. Keep tables via SeparateDatabaseAndState.
• P2: move posthog/cdp/ into products/cdp/backend/ (~17.7k LOC: ~12.9k templates, ~4.7k core including validation.py and filters.py that workflows depends on). Mechanical but wide: 129 importing files. This is a move only, not isolation.
• P3 (optional): move frontend/src/scenes/data-pipelines and frontend/src/scenes/hog-functions into product frontends. Organization win, no backend-CI effect.

Out of scope

• cdp isolation. 75 inbound importers, with Plugin/PluginConfig used throughout core. Needs its own scan after P2 and likely the facade-first, team-sliced path. Plan separately.
• The integration platform (posthog/models/integration.py and friends, 344 importers). Shared platform infrastructure used by many teams; it stays in posthog/.
• nodejs/src/cdp. Separate service, orthogonal to the Django CI matrix.

Verification

• Per PR: tach check --dependencies --interfaces, lint-imports, hogli product:lint workflows, hogli product:maturity workflows.
• After W5: open a trivial workflows-internals PR and confirm turbo-discover reports run_legacy=false with only workflows in the matrix.