Fix: Add missing checksum field to file_meta_t and file_task_t structs

The file_meta_t and file_task_t structs were missing the checksum field
that was being used in both client and server code, causing a compilation
error. Added uint64_t checksum to both structs and updated wq_push
declaration to match implementation.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

Makefile

@@ -0,0 +1,20 @@
+CC = gcc
+CFLAGS = -Wall -Wextra -O3 -D_GNU_SOURCE -pthread
+LDFLAGS = -lpthread
+TARGETS = fastsync_server fastsync_client
+
+all: $(TARGETS)
+
+fastsync_server: src/server.o src/utils.o
+	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
+fastsync_client: src/client.o src/utils.o
+	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
+src/%.o: src/%.c src/common.h
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+clean:
+	rm -f src/*.o $(TARGETS)
+
+.PHONY: all clean