build(Makefile): enable parallel make -j gen with correct dependency graph (#22612)

`make gen` could not run with `-j` because inter-target dependency edges
were missing. Multiple recipes compile `coderd/rbac` (which includes
generated files like `object_gen.go`), and without explicit ordering,
parallel runs produced syntax errors from mid-write reads.

Three main changes:

**Dependency graph fixes** declare the compile-time chain through
`coderd/rbac` so that `object_gen.go` is written before anything that
imports it is compiled. The DB generation targets use a GNU Make 4.3+
grouped target (`&:`) so Make knows `generate.sh` co-produces
`querier.go`, `unique_constraint.go`, `dbmetrics`, and `dbauthz` in a
single invocation. `SKIP_DUMP_SQL=1` avoids re-entrant `make` inside
`generate.sh` when the Makefile already guarantees `dump.sql` is fresh.

**`scripts/atomicwrite` package** replaces `os.WriteFile` in all gen
scripts with a temp-file-in-same-dir + rename pattern, preventing
interrupted runs from leaving partial files.

**`.PRECIOUS` and shell atomic writes** protect git-tracked generated
files from Make's default delete-on-error behavior. Since these files
are committed, deletion is worse than staleness -- `git restore` is the
recovery path.

CI now runs `make -j --output-sync -B gen` (~32s, down from ~85s
serial).

| Scenario                          | Before             | After    |
|-----------------------------------|--------------------|----------|
| `make gen` (serial)               | 95s                | 95s      |
| `make -j gen` (parallel)          | race error         | **22s**  |
| CI `make -j --output-sync -B gen` | forced serial ~85s | **~32s** |

scripts/metricsdocgen/main.go

@@ -13,6 +13,8 @@ import (
 	dto "github.com/prometheus/client_model/go"
 	"github.com/prometheus/common/expfmt"
 	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/scripts/atomicwrite"
 )
 
 var (
@@ -186,13 +188,7 @@ func updatePrometheusDoc(doc []byte, metricFamilies []*dto.MetricFamily) ([]byte
 }
 
 func writePrometheusDoc(doc []byte) error {
-	// G306: Expect WriteFile permissions to be 0600 or less
-	/* #nosec G306 */
-	err := os.WriteFile(prometheusDocFile, doc, 0o644)
-	if err != nil {
-		return err
-	}
-	return nil
+	return atomicwrite.File(prometheusDocFile, doc)
 }
 
 func sortedKeys(m map[string]struct{}) []string {