Add embedded Vue SPA admin panel with go:embed

This is the complete fix for the missing /admin/ route and frontend serving:

Backend:
- internal/web/web.go: new package with go:embed for web/dist/
- internal/api/router.go: add routes for /admin/, /admin/*, /assets/*
- internal/db/db.go: fix SQLite DSN parsing (sqlite:///path -> path)

Build system:
- Makefile: new 'embed-prep' target copies web/dist to internal/web/dist
- make build now runs embed-prep -> frontend/build automatically

Deployment:
- deploy/llamalink.service: remove invalid --host/--port flags,
  add EnvironmentFile=/etc/llamalink/env

Verified:
- /health returns 200
- /admin/ serves Vue SPA HTML
- /assets/* serves CSS and JS files from embedded FS
- sqlite:///./llamalink.db works correctly

Makefile

@@ -24,12 +24,19 @@ GOLINT=golangci-lint
 # Default target
 all: deps build
 
-## build: Build the binary
-build:
+## build: Build the binary (includes frontend embed)
+build: embed-prep
 	@echo "Building $(BINARY)..."
 	@mkdir -p $(BUILD_DIR)
 	$(GOBUILD) -o $(BUILD_DIR)/$(BINARY) ./cmd/llamalink
 
+## embed-prep: Copy web/dist into internal/web/dist for go:embed
+embed-prep: frontend/build
+	@echo "Preparing embedded frontend..."
+	@rm -rf internal/web/dist
+	@mkdir -p internal/web/dist
+	@cp -r web/dist/* internal/web/dist/
+
 ## run: Build and run
 run: build
 	@echo "Running..."
@@ -63,6 +70,7 @@ deps:
 ## clean: Remove build artifacts
 clean:
 	rm -rf $(BUILD_DIR) $(DIST_DIR)
+	rm -rf internal/web/dist
 	rm -f coverage.out
 	rm -f *.db