R-361: the safety dump destroyed the app's own database backup

writeSafetyDump called DumpOne into the app's OWN unit dir and renamed the result
to pre-restore-* afterwards. DumpOne writes <stack>-<dbtype>.sql - the app's
canonical dump - so every safety dump overwrote the app's real backup and then
moved it away, leaving the app with no database backup until the next nightly
run. A local restore-from-unit in that window tells the customer the app never
had a database.

The comment beside it asserted the rename meant it 'can never overwrite the app's
real dump'. False as written, and believed for four months. Measured live before
the fix: docmost and bookstack each held only pre-restore-* files and no
canonical dump.

DumpOneTo takes the final path and derives its own .tmp from it. DumpOne keeps
its signature and calls it with the canonical name. writeSafetyDump asks for its
own name directly; the rename is gone; the comment now states the invariant and
how it is enforced.

db_dumps no longer lists the undo copies. All three consumers of Manifest.DBDumps
were grepped and named - all inside recovery_unit.go, none reads it for recovery.
The files are neither deleted nor hidden.

Tests 1485 -> 1493. FIVE red-proofs, TWO PASSED first time and both are reported:
the behavioural tests inject the dump seam so a mutation inside DumpOneTo was
invisible, and 1.3 had no test at all. Guards added at the layer each defect
lives in; both mutations then convicted.

controller/internal/backup/r355_safetydump_test.go

@@ -34,15 +34,15 @@ func TestR355_SafetyDumpIsTakenForTheCorrectlyAttributedApp(t *testing.T) {
 			{StackName: "paperless-ngx", DBType: DBTypePostgres, ContainerName: "paperless-postgres", ContainerID: "cid"},
 		}, nil
 	}
-	m.safetyDumpFn = func(ctx context.Context, db DiscoveredDB, dumpDir string) DumpResult {
-		p := filepath.Join(dumpDir, string(db.StackName)+"-"+string(db.DBType)+".sql")
-		if err := os.MkdirAll(dumpDir, 0o755); err != nil {
+	m.safetyDumpFn = func(ctx context.Context, db DiscoveredDB, finalPath string) DumpResult {
+		// R-361: the seam takes the FINAL PATH now.
+		if err := os.MkdirAll(filepath.Dir(finalPath), 0o755); err != nil {
 			t.Fatal(err)
 		}
-		if err := os.WriteFile(p, []byte("-- 72 tables\n"), 0o644); err != nil {
+		if err := os.WriteFile(finalPath, []byte("-- 72 tables\n"), 0o644); err != nil {
 			t.Fatal(err)
 		}
-		return DumpResult{DB: db, FilePath: p, Size: 13}
+		return DumpResult{DB: db, FilePath: finalPath, Size: 13}
 	}
 
 	// v0.220.0 (R-379): writeSafetyDump returns the SET it wrote, so a rollback can re-apply EVERY