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-003-uow-context-injection.md

@@ -0,0 +1,48 @@
+# ADR-003: Unit of Work via Context Injection
+
+**Status:** Accepted
+**Date:** 2026-03-18
+
+## Context
+
+Database transactions must span multiple repository calls without requiring each repository method to accept a `Tx` parameter explicitly. Passing `Tx` as a parameter would leak transaction concepts into repository method signatures and force every call site to decide whether it is inside a transaction.
+
+An alternative is ambient transaction state stored in a thread-local or goroutine-local variable, but Go has no such construct, and package-level state would break concurrent use.
+
+## Decision
+
+The active transaction is stored in the request `context.Context` under an unexported key type `ctxTxKey{}`:
+
+```go
+type ctxTxKey struct{}
+```
+
+`UnitOfWork.Do` begins a transaction, injects it into the context, and calls the user-supplied function with the enriched context:
+
+```go
+ctx = context.WithValue(ctx, ctxTxKey{}, tx)
+fn(ctx)
+```
+
+`Client.GetExecutor(ctx)` checks the context for an active transaction first:
+
+```go
+if tx, ok := ctx.Value(ctxTxKey{}).(Executor); ok {
+    return tx
+}
+// fall back to pool
+```
+
+If there is no active transaction, `GetExecutor` returns the pool. This means repository code uses `db.GetExecutor(ctx)` uniformly and is agnostic about whether it is inside a transaction.
+
+`Tx.Commit(ctx)` and `Tx.Rollback(ctx)` both accept `ctx` — this is supported by `pgx.Tx` and matches the overall pgx API convention.
+
+On function error, `UnitOfWork.Do` calls `Rollback` and returns the original error. Rollback failures are logged but do not replace the original error.
+
+## Consequences
+
+- **Positive**: Repository methods need only `ctx context.Context` and `db postgres.Client`; they do not need a separate `Tx` parameter.
+- **Positive**: Nesting `UnitOfWork.Do` calls is safe — the inner call will pick up the already-injected transaction from the context, so a single transaction spans all nested calls. (pgx savepoints are not used; the outer transaction is reused.)
+- **Positive**: The unexported `ctxTxKey{}` type prevents collisions with other packages that store values in the context.
+- **Negative**: The transaction is invisible from a type-system perspective — there is no way to statically verify that a function is called inside a `UnitOfWork.Do`. Violations are runtime errors, not compile-time errors.
+- **Negative**: Passing a context that carries a transaction to a goroutine that outlives the `UnitOfWork.Do` call would use a closed transaction. Callers must not spawn goroutines from inside the `Do` function that outlive `Do`.