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/dbgen/constraint.go

@@ -11,6 +11,8 @@ import (
 
 	"golang.org/x/tools/imports"
 	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/scripts/atomicwrite"
 )
 
 type constraintType string
@@ -135,7 +137,7 @@ const (
 	if err != nil {
 		return err
 	}
-	return os.WriteFile(outputPath, data, 0o600)
+	return atomicwrite.File(outputPath, data)
 }
 
 // generateUniqueConstraints generates the UniqueConstraint enum.

scripts/dbgen/main.go

@@ -17,6 +17,8 @@ import (
 	"github.com/dave/dst/decorator/resolver/guess"
 	"golang.org/x/tools/imports"
 	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/scripts/atomicwrite"
 )
 
 var (
@@ -245,7 +247,7 @@ func orderAndStubDatabaseFunctions(filePath, receiver, structName string, stub f
 	if err != nil {
 		return xerrors.Errorf("process imports: %w", err)
 	}
-	return os.WriteFile(filePath, data, 0o600)
+	return atomicwrite.File(filePath, data)
 }
 
 // compileFuncDecl extracts the function declaration from the given code.