fix: single conversion at a time

See the notes for a full explanation, but basically
Kobo is making simultaneous requests and the
files were conflicting because there were 2 being
downloaded / converted at the same time which
resulted in unreliable behavior.

The solution is to protect the conversion
section to allow a single conversion to complete
before allowing the next one to start.

notes.md

@@ -0,0 +1,18 @@
+Kobo Browser Quirks
+
+Apparently the browser is using a very old version of WebKit.
+
+- Doesn't support `fetch`
+    - No HTMX
+- Doesn't support `secure` or `httpOnly` cookies
+    - They just silently fail to be set with these flags
+- Makes 2 parallel requests whenever an `<a>` link is clicked
+    - This doesn't seem to apply to URL bar navigation
+    - This poses issues when a request is not idempotent. Need to figure out a solution
+      for these cases... For example when converting to Kepub, we encounter failures
+      for the second requests due to file conflicts / deletions happening at the same time.
+      I've fixed this by locking the conversion to a single request at a time with a mutex,
+      but we still do the conversion twice, just one after the other.
+      I was planning on creating an OPDS interface for OpenBooks but this will make all
+      search / download requests send twice which is no good. The fix isn't as simple
+      in that case.