Adopt golib/pg for migrations and pool construction (#125)

## Why

The Postgres schema was a ~180-line DDL blob inlined in `internal/database/postgres.go` and re-executed on every start. It could only grow — each change appended another `ALTER TABLE ... ADD COLUMN IF NOT EXISTS` — and nothing recorded what had ever been applied. `golib/pg` already owns that mechanic for the estate (arrproxy#10), so artifactapi keeps the SQL and hands over the machinery.

## How

- Depend on `git.unkin.net/unkin/golib` v0.1.0.
- Move the DDL **verbatim** into `migrations/0001_init.sql`, embedded by the new `migrations` package, and build the pool with `pg.NewMigrated` (`LockName: "artifactapi-migrations"`). The runner takes a cluster-wide advisory lock the old blob never took, so replicas starting together queue instead of racing each other through the DDL.
- **No-op on the live database.** Production has the schema but no `schema_migrations`, so its first start on this build re-runs `0001`. Every statement is `IF NOT EXISTS`-guarded, so that run changes nothing and only lands the tracking row. `TestMigratingAnAlreadyPopulatedSchemaIsANoOp` drops the row from a migrated database and asserts exactly that; `TestMigrationsAreIdempotent` keeps future migrations additive and idempotent.
- **Env var contract unchanged.** `config.DatabaseDSN` stays the DSN source rather than `pg.DSNFromEnv`: the variable names match, but golib deliberately has no default user or database name, and artifactapi documents and ships `DBUSER`/`DBNAME` defaults of `artifacts`.
- Drift guards: the embedded set is asserted equal to `migrations/`, and the derived advisory key is pinned so a rename cannot silently let two builds migrate at once.
- Plumb `GOPRIVATE=git.unkin.net` for the first cross-repo Go dependency — exported by the `Makefile`, set in the `Dockerfile` and both woodpecker Go steps, documented in the README.

`database.New`'s signature and every caller are untouched.

Transitive bumps come with golib's `go.mod` (testcontainers-go 0.42 → 0.44, otel, x/crypto, x/net); `go build`, `go vet`, `golangci-lint`, `go test -race ./pkg/... ./internal/...` and `pre-commit run --all-files` are clean.

Reviewed-on: #125
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

Makefile

@@ -6,6 +6,11 @@ VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo "0.0
 GO_VERSION_REQUIRED := 1.23
 GO_VERSION_ACTUAL := $(shell go version | sed 's/go version go\([0-9]*\.[0-9]*\).*/\1/')
 
+# git.unkin.net modules (golib) are fetched straight from Gitea, never via the
+# public proxy or sum db, which have no view of them. Exported here rather than
+# written with `go env -w`, so a fresh checkout needs no machine-local setup.
+export GOPRIVATE := git.unkin.net
+
 check-go:
 	@if [ "$$(printf '%s\n%s' "$(GO_VERSION_REQUIRED)" "$(GO_VERSION_ACTUAL)" | sort -V | head -1)" != "$(GO_VERSION_REQUIRED)" ]; then \
 		echo "ERROR: Go >= $(GO_VERSION_REQUIRED) required, found $(GO_VERSION_ACTUAL)"; exit 1; \