fix(docker): rebuild kepubify with Go 1.26.3, fix read-only fs chmod

Replaced pre-compiled kepubify binaries (built with Go 1.17.8, 53
HIGH/CRITICAL CVEs) with binaries rebuilt from source using Go 1.26.3
and golang.org/x/text upgraded to v0.3.8, resolving all fixable CVEs.

Also fixed KepubifyBinaryService incorrectly falling back to downloading
the binary on every startup in read-only containers. chmod() was throwing
EROFS, and the single catch block treated it as a missing file. Split
stat and chmod into separate try/catch so only a missing-file error
triggers the download fallback.

.gitattributes

@@ -0,0 +1 @@
+server/bin/kepubify/* binary