separate media and database paths

The primary motivation for this change is to support databases that
don't take a path, e.g. out of process databases.

This configuration structure leaves the door open for other media
storage mechanisms in the future, such as S3.

It's also structured to avoid `#[serde(flatten)]` so that we can use
`#[serde(deny_unknown_fields)]`.

src/config.rs

@@ -44,7 +44,6 @@ pub(crate) struct Config {
 
     pub(crate) server_discovery: ServerDiscovery,
     pub(crate) database: DatabaseConfig,
-    #[serde(default)]
     pub(crate) media: MediaConfig,
     #[serde(default)]
     pub(crate) federation: FederationConfig,
@@ -74,13 +73,27 @@ pub(crate) struct Config {
     pub(crate) emergency_password: Option<String>,
 }
 
-#[derive(Debug, Deserialize, Default)]
+#[derive(Debug, Deserialize)]
 #[serde(deny_unknown_fields)]
 pub(crate) struct MediaConfig {
+    pub(crate) backend: MediaBackendConfig,
+
     #[serde(default)]
     pub(crate) allow_unauthenticated_access: bool,
 }
 
+#[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields, tag = "type", rename_all = "snake_case")]
+pub(crate) enum MediaBackendConfig {
+    Filesystem(MediaFilesystemConfig),
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub(crate) struct MediaFilesystemConfig {
+    pub(crate) path: PathBuf,
+}
+
 #[derive(Debug, Deserialize)]
 #[serde(deny_unknown_fields, default)]
 pub(crate) struct CacheConfig {

src/service/globals.rs

@@ -39,6 +39,7 @@ use trust_dns_resolver::TokioAsyncResolver;
 
 use crate::{
     api::server_server::FedDest,
+    config::{MediaBackendConfig, MediaFilesystemConfig},
     observability::FilterReloadHandles,
     service::media::MediaFileKey,
     services,
@@ -602,10 +603,11 @@ impl Service {
     }
 
     pub(crate) fn get_media_folder(&self) -> PathBuf {
-        let mut r = PathBuf::new();
-        r.push(self.config.database.path.clone());
-        r.push("media");
-        r
+        let MediaBackendConfig::Filesystem(MediaFilesystemConfig {
+            path,
+        }) = &self.config.media.backend;
+
+        path.clone()
     }
 
     pub(crate) fn get_media_file(&self, key: &MediaFileKey) -> PathBuf {