feat(postgres): initial stable release v0.9.0

pgx v5-native PostgreSQL client with launcher lifecycle, health check, unit-of-work via context injection, and structured error mapping.

What's included:
- Executor / Tx / Client / Component interfaces using pgx native types (pgconn.CommandTag, pgx.Rows, pgx.Row)
- New(logger, cfg) constructor; pgxpool initialised in OnInit
- Config struct with env-tag support for all pool tuning parameters
- UnitOfWork via context injection; GetExecutor(ctx) returns active Tx or pool
- HandleError mapping pgerrcode constants to xerrors codes (AlreadyExists, InvalidInput, NotFound, Internal)
- health.Checkable at LevelCritical; HealthCheck delegates to pgxpool.Ping

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

docs/adr/ADR-002-local-executor-interface.md

@@ -0,0 +1,36 @@
+# ADR-002: Local Executor Interface
+
+**Status:** Accepted
+**Date:** 2026-03-18
+
+## Context
+
+The `Executor` interface — the common query interface shared by the connection pool and an active transaction — must be defined somewhere. Earlier iterations of this codebase placed it in a shared `dbutil` package that both `postgres` and `mysql` imported. This created a cross-cutting dependency: every database module depended on `dbutil`, and `dbutil` had to make choices (e.g., which type system to use) that were appropriate for only one of them.
+
+`dbutil` was eliminated as part of the monorepo refactor (see `plan/decisions.md`).
+
+## Decision
+
+The `Executor` interface is defined locally inside the `postgres` package:
+
+```go
+type Executor interface {
+    Exec(ctx context.Context, sql string, args ...any) (pgconn.CommandTag, error)
+    Query(ctx context.Context, sql string, args ...any) (pgx.Rows, error)
+    QueryRow(ctx context.Context, sql string, args ...any) pgx.Row
+}
+```
+
+The `mysql` package defines its own separate `Executor` using `database/sql` types. The two are not interchangeable by design — they represent different type systems.
+
+`Tx` extends `Executor` with `Commit(ctx context.Context) error` and `Rollback(ctx context.Context) error`. `Client` provides `GetExecutor`, `Begin`, `Ping`, and `HandleError`. `Component` composes `Client`, `launcher.Component`, and `health.Checkable`.
+
+Repository code in application layers should depend on `postgres.Executor` (or the higher-level `postgres.Client`) — not on the concrete `*pgxpool.Pool` or `pgTx` types.
+
+## Consequences
+
+- **Positive**: No shared `dbutil` dependency. Each database module owns its interface and can evolve it independently.
+- **Positive**: The interface methods use pgx-native types, so there is no impedance mismatch between the interface and the implementation.
+- **Positive**: Mocking `postgres.Executor` in tests requires only implementing three methods with pgx return types — no wrapper types needed.
+- **Negative**: If a project uses both `postgres` and `mysql`, neither module's `Executor` is compatible with the other. Cross-database abstractions must be built at the application domain interface layer, not by sharing a common `Executor`.
+- **Note**: `pgComponent` itself also implements `Executor` directly (forwarding to the pool), which means a `*pgComponent` can be used wherever an `Executor` is expected without calling `GetExecutor`. This is intentional for ergonomics in simple cases where no transaction management is needed.