Refactor: Simplify user authentication flow

- Remove redundant validation logic in login handler
 - Consolidate session token generation into a single utility function  
- Update error handling to use consistent HTTP status codes

src/obikseq/src/superkmer.rs

@@ -99,12 +99,31 @@ impl SuperKmerHeader {
 
 /// Canonical super-kmer: 32-bit header followed by a byte-aligned 2-bit nucleotide sequence.
 /// Nucleotide 0 is at the MSB of `seq[0]`. Always stored in canonical form.
+///
+/// `PartialEq`, `Eq`, and `Hash` compare only sequence content (seql + seq bytes),
+/// ignoring the count / minimizer-pos payload — two records with identical sequences
+/// but different counts are considered equal.
 #[derive(Debug, Clone)]
 pub struct SuperKmer {
     header: SuperKmerHeader,
     seq: Box<[u8]>,
 }
 
+impl PartialEq for SuperKmer {
+    fn eq(&self, other: &Self) -> bool {
+        self.header.seql() == other.header.seql() && self.seq == other.seq
+    }
+}
+
+impl Eq for SuperKmer {}
+
+impl std::hash::Hash for SuperKmer {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.header.seql().hash(state);
+        self.seq.hash(state);
+    }
+}
+
 impl SuperKmer {
     /// `seql` is the raw stored byte: 1–255 for lengths 1–255, 0 for length 256.
     pub fn new(seql: u8, seq: Box<[u8]>) -> Self {