media: Use ETag and If-None-Match

Note: this requires a reindex of the media database. For the default
CAS backend, use the following:

```bash
for i in */*/*/*/*.json; do
    etag="$(echo $i | sed -e 's/\///g' -e 's/\.json$//')";
    mv "$i" "$i.bak"
    cat "$i.bak" | jq '. + { "etag": '\""$etag"\"'}' > "$i"
    rm "$i.bak"
done
```

This change is backwards compatible, but caching headers won't be
emitted without etags present in the metadata.

Actual etags are backend-specific and might differ from backend to
backend.

1 files changed, 4 insertions, 1 deletions

diff --git a/kittybox-rs/src/media/storage/mod.rs b/kittybox-rs/src/media/storage/mod.rs
index b34da88..4ef7c7a 100644
--- a/kittybox-rs/src/media/storage/mod.rs
+++ b/kittybox-rs/src/media/storage/mod.rs
@@ -17,6 +17,8 @@ pub struct Metadata {
     pub filename: Option<String>,
     /// The recorded length of the file.
     pub length: Option<NonZeroUsize>,
+    /// The e-tag of a file. Note: it must be a strong e-tag, for example, a hash.
+    pub etag: Option<String>,
 }
 impl From<&Field<'_>> for Metadata {
     fn from(field: &Field<'_>) -> Self {
@@ -25,7 +27,8 @@ impl From<&Field<'_>> for Metadata {
                 .map(|i| i.to_owned()),
             filename: field.file_name()
                 .map(|i| i.to_owned()),
-            length: None
+            length: None,
+            etag: None,
         }
     }
 }