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

Makefile

@@ -25,11 +25,25 @@ $(MKDOCS): $(VENV)/bin/activate
 
 # ── obikmer binary ───────────────────────────────────────────────────────────
 
+PROFILE_FILE := profile.pb
+PPROF_PORT   := 8081
+
 .PHONY: obikmer
 obikmer:
 	cargo build --release --manifest-path $(CARGO_DIR)/Cargo.toml --bin obikmer
+
+.PHONY: debug
 debug:
 	cargo build --manifest-path $(CARGO_DIR)/Cargo.toml --bin obikmer
+
+.PHONY: profile-build
+profile-build:
+	cargo build --release --manifest-path $(CARGO_DIR)/Cargo.toml \
+	            --bin obikmer --features obikmer/profiling
+
+.PHONY: profile-view
+profile-view:
+	go tool pprof -http=127.0.0.1:$(PPROF_PORT) $(PROFILE_FILE)
 # ── documentation ─────────────────────────────────────────────────────────────
 
 .PHONY: doc