Replace serde_json with postcard for cache serialization

Cargo.toml

@@ -20,8 +20,8 @@ path = "src/lib.rs"
 
 [features]
 default = ["cache", "tokenizer", "colored-output"]
-# Translation cache with sled DB
-cache = ["dep:sled", "dep:sha2", "dep:hex"]
+# Translation cache with sled DB + postcard binary serialization
+cache = ["dep:sled", "dep:sha2", "dep:hex", "dep:postcard"]
 # Claude tokenizer for precise token counting
 tokenizer = ["dep:claude-tokenizer"]
 # Colored terminal output
@@ -46,6 +46,7 @@ fastrand = "2"  # Lightweight RNG for retry jitter
 sled = { version = "0.34", optional = true }
 sha2 = { version = "0.10", optional = true }
 hex = { version = "0.4", optional = true }
+postcard = { version = "1", features = ["alloc"], optional = true }
 
 # Optional: Colored output
 colored = { version = "2", optional = true }

src/cache.rs

@@ -176,7 +176,7 @@ mod cache_impl {
         /// Get cached translation if available and not expired
         pub fn get(&self, key: &str) -> Option<CacheEntry> {
             match self.db.get(key) {
-                Ok(Some(bytes)) => match serde_json::from_slice::<CacheEntry>(&bytes) {
+                Ok(Some(bytes)) => match postcard::from_bytes::<CacheEntry>(&bytes) {
                     Ok(entry) => {
                         let now = Utc::now().timestamp();
                         let ttl_secs = self.config.ttl_days as i64 * 24 * 60 * 60;
@@ -190,6 +190,7 @@ mod cache_impl {
                         }
                     }
                     Err(_) => {
+                        let _ = self.db.remove(key);
                         CACHE_MISSES.fetch_add(1, Ordering::Relaxed);
                         None
                     }
@@ -203,7 +204,7 @@ mod cache_impl {
 
         /// Store translation in cache
         pub fn put(&self, key: &str, entry: &CacheEntry) {
-            if let Ok(bytes) = serde_json::to_vec(entry) {
+            if let Ok(bytes) = postcard::to_allocvec(entry) {
                 let entry_size = bytes.len();
                 let _ = self.db.insert(key, bytes);