feat(plugins): implement plugin specific storage (#5839)

* implement storage api/hooks

* fine. if the error messages are different, just match error

* rename storageMount

* add independent plugin test

* round 2

* .-.

* one more copypasta fail

* docs(plugins): document the Storage host service

The README is the plugin author reference and every other host service has a
section there, but Storage had none, so the /storage guest path contract only
existed in code.

Documents the mount point and its backing directory, the manifest permission,
the host function, and the two behaviours an author would otherwise discover
the hard way: there is no size limit, and the directory outlives an uninstall.

* docs(plugins): correct the library filesystem security notes

The README stated in three places that library filesystem access is read-only,
which stopped being true when AllowWriteAccess was added: an administrator can
grant a plugin write access to libraries.

Corrects those and gives the library and storage mounts the same wording for
what the sandbox guarantees, since both now go through the same jail.

* docs(plugins): describe symlink handling in mounts accurately

The security notes claimed paths resolving outside a mount are rejected, which
overstates the jail. Only lexical escapes are: '..' and absolute paths. Creating
symlinks is denied, but symlinks already present are followed and do reach
outside the mount, which is what lets music libraries link folders in from
elsewhere. Both behaviours are pinned by tests in the plugins package.

---------

Co-authored-by: Deluan Quintão <deluan@navidrome.org>

plugins/testdata/test-storage-plugin-2

@@ -0,0 +1 @@
+test-storage-plugin

plugins/testdata/test-storage-plugin/go.mod

@@ -0,0 +1,16 @@
+module test-storage-plugin
+
+go 1.25
+
+require github.com/navidrome/navidrome/plugins/pdk/go v0.0.0
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/extism/go-pdk v1.1.3 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/stretchr/objx v0.5.2 // indirect
+	github.com/stretchr/testify v1.11.1 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
+
+replace github.com/navidrome/navidrome/plugins/pdk/go => ../../pdk/go

plugins/testdata/test-storage-plugin/go.sum

@@ -0,0 +1,14 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/extism/go-pdk v1.1.3 h1:hfViMPWrqjN6u67cIYRALZTZLk/enSPpNKa+rZ9X2SQ=
+github.com/extism/go-pdk v1.1.3/go.mod h1:Gz+LIU/YCKnKXhgge8yo5Yu1F/lbv7KtKFkiCSzW/P4=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.5.2 h1:xuMeJ0Sdp5ZMRXx/aWO6RZxdr3beISkG5/G/aIRr3pY=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
+github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

plugins/testdata/test-storage-plugin/main.go

@@ -0,0 +1,50 @@
+// Test plugin for Storage host function integration tests.
+// Build with: tinygo build -o ../test-storage-plugin.wasm -target wasip1 -buildmode=c-shared ./main.go
+package main
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/navidrome/navidrome/plugins/pdk/go/host"
+	"github.com/navidrome/navidrome/plugins/pdk/go/pdk"
+)
+
+//go:wasmexport call_read
+func callRead() int32 {
+	path := filepath.Join(host.StorageGetStoragePath(), pdk.InputString())
+	content, err := os.ReadFile(path)
+	if err != nil {
+		pdk.SetErrorString("failed to read file: " + err.Error())
+		return 1
+	}
+	pdk.Output(content)
+	return 0
+}
+
+type WriteInput struct {
+	Path     string `json:"path"`
+	Contents string `json:"contents"`
+}
+
+//go:wasmexport call_write
+func callWrite() int32 {
+	var config WriteInput
+	err := pdk.InputJSON(&config)
+
+	if err != nil {
+		pdk.SetErrorString("failed to parse json: " + err.Error())
+		return 1
+	}
+
+	path := filepath.Join(host.StorageGetStoragePath(), config.Path)
+	err = os.WriteFile(path, []byte(config.Contents), 0600)
+	if err != nil {
+		pdk.SetErrorString("failed to write file: " + err.Error())
+		return 1
+	}
+
+	return 0
+}
+
+func main() {}

plugins/testdata/test-storage-plugin/manifest.json

@@ -0,0 +1,11 @@
+{
+  "name": "Test Storage Plugin",
+  "author": "Navidrome Test",
+  "version": "1.0.0",
+  "description": "Test plugin for storage host function",
+  "permissions": {
+    "storage": {
+      "reason": "persistent storage for testing"
+    }
+  }
+}