docs(adr): record Kysely containment decision as ADR-006

docs/adr/ADR-006-kysely-contained-inside-db-adapter.md

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+# ADR-006: Kysely contained inside DB adapter
+
+- Status: Accepted
+- Date: 2026-08-31
+- Deciders: platform stream
+- References: ADR-001, ADR-005, Architecture wiki (sections 1, 4, 9),
+  Engineering-Standards wiki (sections 24, 25, 57)
+
+## Context
+
+EPPP is a modular monolith (ADR-001) that keeps every module boundary
+explicit, versioned and CI-enforced. ADR-005 already records the database
+choice — PostgreSQL 18 as the sole canonical database — and with it the rule
+that the `pg` driver and the Kysely query builder are isolated to
+`packages/database-postgres`, the adapter boundary every database access
+passes through. This ADR is the focused record of where the query builder
+itself may live: the containment decision.
+
+Kysely is a type-safe SQL query builder, not an ORM. It gives typed queries
+and composable expressions on top of `pg`, but it still speaks SQL and a
+dialect, and every package that imports it is coupled to that dialect and to
+the adapter's implementation choices. The workspace already depends on the
+containment: `packages/database-postgres` declares `kysely` 0.29.4 and `pg`
+8.22.0 as its exact-pinned runtime dependencies (its only ones), imports them
+in its source and re-exports the pieces the adapter is built on (E00-S03-T02),
+and is the single workspace package whose source may import the driver — a
+rule locked in by `tests/database-postgres-imports.test.mjs` in the
+`database-postgres-imports` CI job and by the architecture fitness tests
+FIT-010 (browser bundles cannot import server/DB packages) and FIT-011 (UI
+cannot import Kysely/pg).
+
+The dependency direction is already fixed: `database-postgres → core ports →
+Kysely/pg` (Architecture wiki §9.1, Engineering-Standards §24). Domain and
+extension packages talk to the database through core repository/port
+interfaces, never through the query builder. What remains to be recorded is
+that this is a standing architectural decision, not a temporary arrangement:
+Kysely stays inside the adapter, and no other package may grow a Kysely or
+`pg` import surface.
+
+## Decision
+
+EPPP keeps **Kysely contained inside DB adapter**. Kysely is an
+implementation detail of `packages/database-postgres` — the single workspace
+package allowed to import `pg`/Kysely (E00-S03-T02) — and the rest of the
+workspace reaches the database exclusively through the adapter's and core
+ports' APIs. No other workspace package may declare `kysely` or `pg` in its
+manifest or import them directly in its source; the `database-postgres-imports`
+CI gate plus FIT-010/FIT-011 enforce this on every PR. Kysely exists only
+where the adapter lives — it is contained, not distributed, and the containment
+is a package-level rule, not a type-level one.
+
+## Alternatives
+
+- **Kysely imported directly by domain/extension packages** — rejected: it
+  would leak SQL-dialect and query-builder concerns into domain and extension
+  code, defeat the single-owner boundary (E00-S03-T02), make FIT-011
+  impossible to honour, and couple business logic to the adapter's
+  implementation.
+- **A dedicated shared query-layer package** — rejected: it would create a
+  second import surface for the driver stack, split ownership of the dialect,
+  and add a package whose only purpose is to widen the boundary; the adapter
+  already owns the typed surface callers need.
+- **Full ORM (e.g. Prisma/Drizzle) instead of a query builder** — rejected:
+  schema is owned by the adapter's migration chains (ADR-005,
+  Engineering-Standards §25), and an ORM adds code generation and a schema
+  file that would couple domain models to storage; Kysely's typed builder is
+  sufficient and stays contained.
+- **Raw `pg` everywhere / no query builder** — rejected: hand-written SQL
+  loses type safety and composability while still requiring the driver;
+  containing a query builder is strictly better than containing SQL strings.
+- **Adapter re-exports Kysely for other packages to build queries** — rejected
+  (variant of the first alternative): even when routed through the adapter,
+  letting other packages compose Kysely queries would leak the dialect and
+  bypass the port/repository API that keeps domain code storage-agnostic.
+
+## Consequences
+
+- Positive: one owner for the whole driver/query-builder stack — upgrade,
+  dialect and typing decisions live in `packages/database-postgres` and are
+  exact-pinned single-manifest changes, so replacing or upgrading Kysely and
+  `pg` touches exactly one manifest; domain and extension code stays
+  driver-free, so FIT-010/FIT-011 hold; replacing Kysely or the dialect later
+  is a contained change behind the adapter boundary (consistent with the
+  ADR-005 encapsulation).
+- Negative: the adapter's port/repository API must be designed well enough
+  that domain code never needs the query builder — the port API is mandatory,
+  not optional; Kysely conveniences (expression builders, plugin features) are
+  not available to callers outside the adapter; the adapter package grows as
+  the query surface grows.
+- Neutral: Kysely remains a dependency of the adapter package only; contracts
+  between core ports and the adapter are plain TypeScript interfaces, so the
+  containment stays a package-level rule and never becomes a type-level one.
+
+## Operational impact
+
+- `kysely` and `pg` appear in exactly one manifest
+  (`packages/database-postgres/package.json`) and in exactly one package's
+  source; a static scan in `tests/database-postgres-imports.test.mjs`
+  (`database-postgres-imports` CI job) proves it on every PR, including a
+  mutation probe that fails when a driver import is injected elsewhere.
+- Upgrading Kysely or `pg` is a single-manifest change inside the adapter,
+  reviewed under the workspace's dependency/upgrade lane; no other package's
+  manifest or code moves.
+- No new runtime services, no schema changes, no deployment or configuration
+  impact: this ADR is documentation of an already-enforced boundary.
+
+## Revisit trigger
+
+- Revisit this ADR when a non-adapter package demonstrably needs
+  query-builder features that cannot be expressed through the adapter/port
+  API, and there is evidence (per the architecture review gates) that the
+  boundary costs more than containment saves.
+- Revisit if the platform grows additional data stores or engines (shared with
+  the ADR-005 revisit trigger): a second engine may need its own contained
+  adapter, and the query-builder containment rule would extend per adapter.
+- Revisit if the dependency-boundary enforcement changes — for example if
+  FIT-010/FIT-011 or the `database-postgres-imports` gate are relaxed or
+  re-scoped.